ZeroNet sometimes hangs

On log messages like:
`[18:29:16] Site:1F7b27..nQpw Announced types ['ip4'] in mode startup to 6 trackers in 10.001s, errors: ['udp://tracker.leechers-paradise.org:6969'], slow: ['10s+ http://tracker.tordb.ml:6881/announce']`
or
`[18:31:19] Site:1F7b27..nQpw Queried listModifications from: [, , , , ]`
zeronet just hangs a long time (~1-5 mins), without responding to requests

Some more log:
```
[18:31:19] Site:1UDbAD..MkoV [FAILED] 87.8.48.197:15441: {'to': 21, 'cmd': 'response', 'error': 'File invalid'}
[18:31:19] Site:1XdLr6..sbrx [FAILED] 83.162.192.96:15442: {'to': 34, 'cmd': 'response', 'error': 'File invalid'}
[18:31:19] FileServer 114.83.43.45 Connect error: error: [Errno 111] Connection refused in ConnectionServer.py line 139 > Connection.py line 100 > _socket2.py line 231
[18:31:19] FileServer Removing Conn#454 114.83.43.45 [?]...
[18:31:19] FileServer Conn#468 192.210.179.116 [v2] > Crypt out connection using: tls-rsa (server side: False)...
[18:31:19] FileServer Conn#516 536zml6mcwrboffk.onion [?] > Connecting...
[18:31:19] TorManager Creating new socket to 536zml6mcwrboffk.onion:15441
[18:31:19] FileServer 223.148.244.101 Connect error: error: [Errno 111] Connection refused in ConnectionServer.py line 139 > Connection.py line 100 > _socket2.py line 231
[18:31:19] FileServer Removing Conn#484 223.148.244.101 [?]...
[18:31:19] FileServer Conn#517 l3s7qjjuechqwjqq.onion [?] > Connecting...
[18:31:19] TorManager Creating new socket to l3s7qjjuechqwjqq.onion:15441
[18:31:19] Site:1UDbAD..MkoV [FAILED] 146.0.226.141:15441: {'to': 32, 'cmd': 'response', 'error': 'File invalid'}
[18:31:19] FileServer 39.187.165.167 Connect error: error: [Errno 111] Connection refused in ConnectionServer.py line 139 > Connection.py line 100 > _socket2.py line 231
[18:31:19] FileServer Removing Conn#445 39.187.165.167 [?]...
[18:31:19] FileServer Conn#518 114.93.100.3 [?] > Connecting...
[18:31:19] FileServer Conn#519 slxwz7fe6m2volyg.onion [?] > Connecting...
[18:31:19] TorManager Creating new socket to slxwz7fe6m2volyg.onion:15441
[18:31:19] FileServer 114.252.75.160 Connect error: error: [Errno 113] No route to host in ConnectionServer.py line 139 > Connection.py line 100 > _socket2.py line 231
[18:31:19] FileServer Removing Conn#433 114.252.75.160 [?]...
[18:31:19] FileServer Conn#520 lfeyaddtybbkwoai.onion [?] > Connecting...
[18:31:19] TorManager Creating new socket to lfeyaddtybbkwoai.onion:15441
[18:31:19] Site:1UDbAD..MkoV [FAILED] 81.101.99.227:15441: {'to': 21, 'cmd': 'response', 'error': 'File invalid'}
[18:31:19] Site:1F7b27..nQpw Queried listModifications from: [, , , , ]
```

possible fix: use multi-threading