Better handling of JOIN/PART events.

[nolanlum]

• Channels joined by a particular IRC client connection are now tracked at that level. This way, if a client locally PARTs a Slack channel (even though the state does not update on the Slack side), we can allow the client to re-join a channel (once) without having to reconnect. Previously there was no way to re-join a channel you left on an IRC client (we would never send the JOIN echo since we checked joined channels against Slack).
• Add handling for channel_joined and channel_left. This information was duplicated by member_joined_channel and member_left_channel, with the sole exception of channel archiving, which does not generate member_left_channel. We now ignore the member_* variant for the self user.
• Replace chan interface{} and map[x]interface{} with struct{} lmbo.
• Ignore known and unimportant unmarshalling errors.

[Koshroy]

Should we use errors.Is here?