Delay sending JOIN command until after MOTD finishes for IRC bot

Summary: Do JOIN in the protocol handler, after we receive 376 ("end of motd").

Test Plan: Ran bot, it joined a channel after receieving a 376 command.

Reviewers: moos3, codeblock, aran, jungejason, tuomaspelkonen

Reviewed By: moos3

CC: aran, moos3

Differential Revision: 855