Add a connected _client to the set, or do nothing
if the _client is already in the set.
The _client is automatically removed
if it is disconnected inside an event
callback registered on the _client.
If the _client is disconnected outside
the event loop, it is the caller's
responsibility to call IrcEventLoop.remove.
Add a connected _client to the set, or do nothing if the _client is already in the set.
The _client is automatically removed if it is disconnected inside an event callback registered on the _client. If the _client is disconnected outside the event loop, it is the caller's responsibility to call IrcEventLoop.remove.