Replace CIRCNetwork::FindClient() with FindClients()

FindClient() is not enough, because there are no restrictions to used
identifiers. They don't necessarily need to be unique, and the same
identified client might re-connect meanwhile a ghost connection is
still hanging there.
Update Csocket to b5185dc

> Fix IsConnected() to return false when Disconnected() gets called
Say hello to a channel filter module for identified clients

The channel filter module maintains client specific channel lists for
identified clients. A typical use case is to have a subset of channels
visible for a mobile client.

The module detects identified clients automatically, and starts
maintaining client specific lists of channels. When an identified
client connects ZNC first time, all channels are joined. The list of
channels is automatically updated when the identified client joins and
parts channels. Next time the identified client connects, it joins the
channels it had visible from the last session.