Fix following comment and recommandations:
- Use DateTime? type instead of int64?
- check the computed delay value against sane values
- use a HashMap to keep track of timers and cancel them on connection
closed
- add equals and hash funcs to XmppStream to use with the HashMap
- rename the callback to reflect its meaning
- test the TURN server conf before STUN default server
Add a field in `Xmpp.Xep.ExternalServiceDiscovery` to keep track
of the `expires` TURN service value and use it (divided by 2) to
restart periodically the external services discovery.
Add libnice as a plugin. If it is present, use libnice to enumerate
local IP addresses and listen on them to support direct connections for
Jingle SOCKS5.
Tested with Conversations and Gajim.
Created the nice.vapi file using
```
vapigen --library nice --pkg gio-2.0 --metadatadir metadata /usr/share/gir-1.0/Nice-0.1.gir
```
