First, this patch moves tgChatId registry value in the channel scope, so
each channel can have a different ID. Then
- a message arriving via IRC is sent to the appropriate chat if tgChatId
  is set for that channel.
- a message arriving via Telegram is sent to a IRC channel by searching
  in all the channels for the one with a matching tgChatId.

As a result, the doJoin hack is no longer necessary (since we already
have all relevant chat IDs)