Add a limit to the number of connections made to irc servers

New configuration setting: conf.max_server_conns.

Limit is ignored if there is a WEBIRC password set for the server or if the server is running
in restricted mode.
Mergability with development branch
Also check ip_as_username config when applying per-server limit
Connection limiting
Merge branch 'development' of github.com:prawnsalad/KiwiIRC into development