prune users from _cliconns when they're on the far side of a split

[jesopo]

this involves receiving a netsplit snote and figuring out what servers are on the far side of the split, then remove any user from _cliconns when they're on an affected server.

the easiest way to do this is to change _cliconns to hold a tuple of (cliconn_id, server_name) (instead of just cliconn_id), then run a /links when there's a netsplit and pop any _cliconn entry from a server we can no longer see