Don't drop server refs when closing channels.

There's no need to clear the server reference - if the channel has
been removed then it will be eligible for GC. If the whole server
is going then the whole group of objects will be eligible for GC.

Update to latest version of findbugs and depend on jsr305 to get
@Nonnull annotation and apply it to the methods which are now
guaranteeing non-null returns.

Change-Id: Ibdcf43e64910440ba5efeb7e090edb72afd858e3
Reviewed-on: http://gerrit.dmdirc.com/2982
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager