Common client connection errors when using server getting started guide

Moving the commentary from https://github.com/aio-libs/aioftp/pull/85 over here.

When I use the Getting started guide to spin up a server, the server isn't accessible from many common clients - particularly these MacOS ones.

### Success cases

✅ - FTP CLI (on Ubuntu)
✅ - Firefox (72.0.2 (64-bit) on MacOS)

On Ubuntu 18.04 I've installed aioftp and am running `python -m aioftp --memory --family ipv4` to start ther server.

#### ftp command line
The client outputs
```
$ ftp 172.16.11.120 2121

Connected to 172.16.11.120.
220 welcome
Name (172.16.11.120:thomasharvey):
230 anonymous login
Remote system type is UNIX.
Using binary mode to transfer files.
```

and on the server I see:
```
aioftp v0.15.0
[17:04:06]: [aioftp.server] serving on 0.0.0.0:2121
[17:06:47]: [aioftp.server] new connection from 172.16.11.120:47164
[17:06:47]: [aioftp.server] 220 welcome
[17:06:48]: [aioftp.server] USER thomasharvey
[17:06:48]: [aioftp.server] 230 anonymous login
[17:06:48]: [aioftp.server] SYST
[17:06:48]: [aioftp.server] 215 UNIX Type: L8
```

#### Firefox
I can connect OK, the server shows:

```
aioftp v0.15.0
[17:15:36]: [aioftp.server] serving on 0.0.0.0:2121
[17:15:52]: [aioftp.server] new connection from 172.16.11.4:63333
[17:15:52]: [aioftp.server] 220 welcome
[17:15:52]: [aioftp.server] USER anonymous
[17:15:52]: [aioftp.server] 230 anonymous login
[17:15:52]: [aioftp.server] SYST
[17:15:52]: [aioftp.server] 215 UNIX Type: L8
[17:15:52]: [aioftp.server] FEAT
[17:15:52]: [aioftp.server] 502 'feat' not implemented
[17:15:52]: [aioftp.server] PWD
[17:15:52]: [aioftp.server] 257 "/"
[17:15:52]: [aioftp.server] TYPE I
[17:15:52]: [aioftp.server] 200
[17:15:52]: [aioftp.server] PASV
[17:15:52]: [aioftp.server] 227-listen socket created
[17:15:52]: [aioftp.server] 227 (0,0,0,0,172,87)
[17:15:52]: [aioftp.server] CWD /
[17:15:52]: [aioftp.server] 250
[17:15:52]: [aioftp.server] LIST
[17:15:52]: [aioftp.server] 150 list transfer started
[17:15:52]: [aioftp.server] 226 list transfer done
```

### Failure cases

❌ - FTP CLI (on MacOS)
❌ - Transmit (on MacOS)
❌ - Cyberduck (on MacOS)
❌ - Filezilla (on MacOS)
❌ - Finder (on MacOS)

#### Transmit
Client log:
```
**********

Date/Time: 2020-03-03 17:05:57 +0000

1: Transmit 5.6.3 (x86_64) Session Transcript [Version 10.15.2 (Build 19C57)] (03/03/2020, 18:05)
1: LibNcFTP 3.2.3 (July 23, 2009) compiled for UNIX
1: 220: welcome
1: Connected to 172.16.11.120.
1: Cmd: USER anonymous
1: 230: anonymous login
1: Cmd: TYPE A
1: 200:
1: Logged in to 172.16.11.120 as anonymous.
1: Cmd: SYST
1: 215: UNIX Type: L8
1: Cmd: FEAT
1: 502: 'feat' not implemented
1: Cmd: PWD
1: 257: "/"
1: Cmd: PASV
1: Cannot parse PASV response: listen socket created
1: 227: listen socket created
1: (0,0,0,0,137,217)
1: Passive mode refused.
1: Connection falling back to port (PORT) mode.
1: Cmd: PORT 172,16,11,4,244,200
1: 502: 'port' not implemented
1: Cmd: PORT 172,16,11,4,244,201
1: 502: 'port' not implemented
1: Cmd: PORT 172,16,11,4,244,202
1: 502: 'port' not implemented
1: Cmd: PORT 172,16,11,4,244,203
1: 502: 'port' not implemented
1: Canceling…
1: Disconnecting from server…
1: Cmd: QUIT
1: 221: bye
```

And on the server side:
```
aioftp v0.15.0
[17:05:50]: [aioftp.server] serving on 0.0.0.0:2121
[17:05:57]: [aioftp.server] new connection from 172.16.11.4:62663
[17:05:57]: [aioftp.server] 220 welcome
[17:05:57]: [aioftp.server] USER anonymous
[17:05:57]: [aioftp.server] 230 anonymous login
[17:05:57]: [aioftp.server] TYPE A
[17:05:57]: [aioftp.server] 200
[17:05:57]: [aioftp.server] SYST
[17:05:57]: [aioftp.server] 215 UNIX Type: L8
[17:05:57]: [aioftp.server] FEAT
[17:05:57]: [aioftp.server] 502 'feat' not implemented
[17:05:57]: [aioftp.server] PWD
[17:05:57]: [aioftp.server] 257 "/"
[17:05:57]: [aioftp.server] PASV
[17:05:57]: [aioftp.server] 227-listen socket created
[17:05:57]: [aioftp.server] 227 (0,0,0,0,137,217)
[17:05:57]: [aioftp.server] PORT 172,16,11,4,244,200
[17:05:57]: [aioftp.server] 502 'port' not implemented
[17:05:57]: [aioftp.server] PORT 172,16,11,4,244,201
[17:05:57]: [aioftp.server] 502 'port' not implemented
[17:05:57]: [aioftp.server] PORT 172,16,11,4,244,202
[17:05:57]: [aioftp.server] 502 'port' not implemented
[17:05:57]: [aioftp.server] PORT 172,16,11,4,244,203
[17:05:57]: [aioftp.server] 502 'port' not implemented
[17:05:57]: [aioftp.server] QUIT
[17:05:57]: [aioftp.server] 221 bye
[17:05:57]: [aioftp.server] closing connection from 172.16.11.4:62663
```

#### Cyberduck
Client log:
```
2020-03-03 18:09:58.858685+0100 PWD
2020-03-03 18:09:58.916890+0100 220 welcome
2020-03-03 18:09:58.917302+0100 USER anonymous
2020-03-03 18:09:58.921265+0100 230 anonymous login
2020-03-03 18:09:58.921354+0100 FEAT
2020-03-03 18:09:58.944287+0100 502 'feat' not implemented
2020-03-03 18:09:58.944396+0100 SYST
2020-03-03 18:09:58.949611+0100 215 UNIX Type: L8
2020-03-03 18:09:58.951729+0100 PWD
2020-03-03 18:09:58.955885+0100 257 "/"
2020-03-03 18:09:58.956282+0100 STAT /
2020-03-03 18:09:58.960571+0100 502 'stat' not implemented
2020-03-03 18:09:58.960861+0100 CWD /
2020-03-03 18:09:58.964190+0100 250
2020-03-03 18:09:58.964260+0100 TYPE A
2020-03-03 18:09:58.968861+0100 200
2020-03-03 18:09:58.968979+0100 PASV
2020-03-03 18:09:58.993767+0100 227-listen socket created
227 (0,0,0,0,139,73)
2020-03-03 18:09:58.994043+0100 CWD /
2020-03-03 18:09:58.998878+0100 250
2020-03-03 18:09:58.999037+0100 TYPE A
2020-03-03 18:09:59.002920+0100 200
2020-03-03 18:09:59.003037+0100 PASV
2020-03-03 18:09:59.007835+0100 227-listen socket already exists
227 (0,0,0,0,139,73)
```

And on the server side:
```
aioftp v0.15.0
[17:09:55]: [aioftp.server] serving on 0.0.0.0:2121
[17:09:58]: [aioftp.server] new connection from 172.16.11.4:62881
[17:09:58]: [aioftp.server] 220 welcome
[17:09:58]: [aioftp.server] USER anonymous
[17:09:58]: [aioftp.server] 230 anonymous login
[17:09:58]: [aioftp.server] FEAT
[17:09:58]: [aioftp.server] 502 'feat' not implemented
[17:09:58]: [aioftp.server] SYST
[17:09:58]: [aioftp.server] 215 UNIX Type: L8
[17:09:58]: [aioftp.server] PWD
[17:09:58]: [aioftp.server] 257 "/"
[17:09:58]: [aioftp.server] STAT /
[17:09:58]: [aioftp.server] 502 'stat' not implemented
[17:09:58]: [aioftp.server] CWD /
[17:09:58]: [aioftp.server] 250
[17:09:58]: [aioftp.server] TYPE A
[17:09:58]: [aioftp.server] 200
[17:09:58]: [aioftp.server] PASV
[17:09:58]: [aioftp.server] 227-listen socket created
[17:09:58]: [aioftp.server] 227 (0,0,0,0,139,73)
[17:09:58]: [aioftp.server] CWD /
[17:09:58]: [aioftp.server] 250
[17:09:58]: [aioftp.server] TYPE A
[17:09:58]: [aioftp.server] 200
[17:09:58]: [aioftp.server] PASV
[17:09:58]: [aioftp.server] 227-listen socket already exists
[17:09:58]: [aioftp.server] 227 (0,0,0,0,139,73)
```

#### Finder
The server log output:
```
aioftp v0.15.0
[17:10:50]: [aioftp.server] serving on 0.0.0.0:2121
[17:11:06]: [aioftp.server] new connection from 172.16.11.4:62907
[17:11:06]: [aioftp.server] new connection from 172.16.11.4:62908
[17:11:06]: [aioftp.server] 220 welcome
[17:11:06]: [aioftp.server] USER anonymous
[17:11:06]: [aioftp.server] 220 welcome
[17:11:06]: [aioftp.server] USER anonymous
[17:11:06]: [aioftp.server] 230 anonymous login
[17:11:06]: [aioftp.server] 230 anonymous login
[17:11:06]: [aioftp.server] SYST
[17:11:06]: [aioftp.server] SYST
[17:11:06]: [aioftp.server] 215 UNIX Type: L8
[17:11:06]: [aioftp.server] 215 UNIX Type: L8
[17:11:06]: [aioftp.server] PWD
[17:11:06]: [aioftp.server] 257 "/"
[17:11:06]: [aioftp.server] PWD
[17:11:06]: [aioftp.server] 257 "/"
[17:11:06]: [aioftp.server] TYPE I
[17:11:06]: [aioftp.server] TYPE I
[17:11:06]: [aioftp.server] 200
[17:11:06]: [aioftp.server] 200
[17:11:06]: [aioftp.server] CWD /
[17:11:06]: [aioftp.server] CWD /
[17:11:06]: [aioftp.server] 250
[17:11:06]: [aioftp.server] 250
[17:11:06]: [aioftp.server] PASV
[17:11:06]: [aioftp.server] PORT 172,16,11,4,245,189
[17:11:06]: [aioftp.server] 502 'port' not implemented
[17:11:06]: [aioftp.server] 227-listen socket created
[17:11:06]: [aioftp.server] 227 (0,0,0,0,149,49)
[17:11:06]: [aioftp.server] dispatcher caught exception
Traceback (most recent call last):
File "/home/thomasharvey/.pyenv/versions/aioftp/lib/python3.7/site-packages/aioftp/server.py", line 913, in dispatcher
result = task.result()
File "/home/thomasharvey/.pyenv/versions/aioftp/lib/python3.7/site-packages/aioftp/server.py", line 498, in parse_command
raise ConnectionResetError
ConnectionResetError
[17:11:06]: [aioftp.server] closing connection from 172.16.11.4:62907
[17:11:06]: [aioftp.server] dispatcher caught exception
Traceback (most recent call last):
File "/home/thomasharvey/.pyenv/versions/aioftp/lib/python3.7/site-packages/aioftp/server.py", line 913, in dispatcher
result = task.result()
File "/home/thomasharvey/.pyenv/versions/aioftp/lib/python3.7/site-packages/aioftp/server.py", line 498, in parse_command
raise ConnectionResetError
ConnectionResetError
[17:11:06]: [aioftp.server] closing connection from 172.16.11.4:62908
```
And god knows where to look for the finder logs :)

#### Filezilla
The client reports:
```
Status: Connecting to 172.16.11.120:2121...
Status: Connection established, waiting for welcome message...
Status: Plain FTP is insecure. Please switch to FTP over TLS.
Status: Server does not support non-ASCII characters.
Status: Logged in
Status: Retrieving directory listing...
Command: PWD
Response: 257 "/"
Command: TYPE I
Response: 200
Command: PASV
Response: 227-listen socket created
Response: 227 (0,0,0,0,136,203)
Command: LIST
Error: The data connection could not be established: ECONNREFUSED - Connection refused by server
Response: 150 list transfer started
Response: 425 Can't open data connection
Error: Failed to retrieve directory listing
```

While the server outputs:
```
[17:17:54]: [aioftp.server] serving on 0.0.0.0:2121
[17:18:46]: [aioftp.server] new connection from 172.16.11.4:63445
[17:18:46]: [aioftp.server] 220 welcome
[17:18:46]: [aioftp.server] USER anonymous
[17:18:46]: [aioftp.server] 230 anonymous login
[17:18:46]: [aioftp.server] SYST
[17:18:46]: [aioftp.server] 215 UNIX Type: L8
[17:18:46]: [aioftp.server] FEAT
[17:18:46]: [aioftp.server] 502 'feat' not implemented
[17:18:46]: [aioftp.server] PWD
[17:18:46]: [aioftp.server] 257 "/"
[17:18:46]: [aioftp.server] TYPE I
[17:18:46]: [aioftp.server] 200
[17:18:46]: [aioftp.server] PASV
[17:18:46]: [aioftp.server] 227-listen socket created
[17:18:46]: [aioftp.server] 227 (0,0,0,0,136,203)
[17:18:46]: [aioftp.server] LIST
[17:18:46]: [aioftp.server] 150 list transfer started
[17:18:47]: [aioftp.server] 425 Can't open data connection
```