authentication issue with md5 on postgres 11

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* **asyncpg version**: 0.21.0 and lower
* **PostgreSQL version**: 11
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**:
* **Python version**: 3.6.0
* **Platform**: linux 3.10.0
* **Do you use pgbouncer?**: No
* **Did you install asyncpg with pip?**: Yes
* **If you built asyncpg locally, which version of Cython did you use?**:
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**:

> We have the same code. When running with useridAAA with password, it works on some servers, but not on other servers. The useridAAA's password is authenticated with md5 on the postgres. If we switch to useridBBB, which is authenticated as SHA256 on postgres. It works on all servers. It's the same password for useridAAA and useridBBB. What's dictating the hash on the password on the client side?

Below is the error message when couldn't be authenticated.
`DEBUG: Using selector: EpollSelector

DEBUG: Get address info edclpgsd320c.bcbsfl.com:5420, type=

DEBUG: Getting address info edclpgsd320c.bcbsfl.com:5420, type= took 2.792 ms: [(, , 6, '', ('))]

DEBUG: poll 59997.326 ms took 1.820 ms: 1 events

DEBUG: connect to ()

DEBUG: poll 59993.166 ms took 0.141 ms: 1 events

DEBUG: connected to : (<_SelectorSocketTransport fd=6 read=polling write=>, )

DEBUG: poll 59990.760 ms took 0.014 ms: 1 events

INFO: poll 59990.568 ms took 60000.243 ms: 1 events

DEBUG: <_SelectorSocketTransport fd=6 read=polling write=> received EOF

Traceback (most recent call last):

File "asyncdb3c.py", line 52, in

app = loop.run_until_complete(init_app())

File "/usr/lib64/python3.6/asyncio/base_events.py", line 484, in run_until_complete

return future.result()

File "asyncdb3c.py", line 34, in init_app

pool = await asyncpg.create_pool("postgres://useridAAA:------@------.-----.com:5490/xxx01s1?application_name=aaa",min_size=1, max_size=3,max_inactive_connection_lifetime=3)

File "/usr/lib64/python3.6/asyncio/coroutines.py", line 110, in __next__

return self.gen.send(None)

File "/u/i3ye/ppx/lib64/python3.6/site-packages/asyncpg/pool.py", line 398, in _async__init__

await self._initialize()

File "/usr/lib64/python3.6/asyncio/coroutines.py", line 110, in __next__

return self.gen.send(None)

File "/u/i3ye/ppx/lib64/python3.6/site-packages/asyncpg/pool.py", line 426, in _initialize

await first_ch.connect()

File "/usr/lib64/python3.6/asyncio/coroutines.py", line 110, in __next__

return self.gen.send(None)

File "/u/i3ye/ppx/lib64/python3.6/site-packages/asyncpg/pool.py", line 125, in connect

self._con = await self._pool._get_new_connection()

File "/usr/lib64/python3.6/asyncio/coroutines.py", line 110, in __next__

return self.gen.send(None)

File "/u/i3ye/ppx/lib64/python3.6/site-packages/asyncpg/pool.py", line 472, in _get_new_connection

**self._connect_kwargs)

File "/usr/lib64/python3.6/asyncio/coroutines.py", line 110, in __next__

return self.gen.send(None)

File "/u/i3ye/ppx/lib64/python3.6/site-packages/asyncpg/connection.py", line 1727, in connect

max_cacheable_statement_size=max_cacheable_statement_size)

File "/usr/lib64/python3.6/asyncio/coroutines.py", line 110, in __next__

return self.gen.send(None)

File "/u/i3ye/ppx/lib64/python3.6/site-packages/asyncpg/connect_utils.py", line 666, in _connect

connection_class=connection_class)

File "/usr/lib64/python3.6/asyncio/coroutines.py", line 110, in __next__

return self.gen.send(None)

File "/u/i3ye/ppx/lib64/python3.6/site-packages/asyncpg/connect_utils.py", line 642, in _connect_addr

await asyncio.wait_for(connected, timeout=timeout)

File "/usr/lib64/python3.6/asyncio/tasks.py", line 358, in wait_for

return fut.result()

asyncpg.exceptions.ConnectionDoesNotExistError: connection was closed in the middle of operation

`

Start by reproducing the failure with PostgreSQL 11 using the asyncpg.create_pool or asyncpg.connect entry points shown in the traceback, comparing md5 and SHA256-authenticated users. Read the connection path around asyncpg/connect_utils.py and establish the expected client behavior and a reproducible test case; done means the md5 authentication difference is explained or corrected and verified.

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