Automatically attach preclients to an existing session of the account, if any

[progval]

Tested with progval/irctest#304 which fails because of:

• RPL_NOWAWAY is sent to all connections to a session #155
• Decide what to do with self messages #156
• a single client sending a QUIT sends a QUIT to every other user

TODO:

• fill the blanks in the fake_log_entry objects (or directly call self.send_now for both AWAY and JOIN+TOPIC+NAMES? either way it's annoying because the idiomatic ways requires a log entry if we want account-tag and the like)
• make it configurable? (per-user or per-server)