[Engine-IRC] Implemented passive channel sync (closes: #787)

Joining channels is now much faster and we no longer need to send WHO
requests for every joined user in a channel.
[Engine-IRC] Take optional target parameter for /mode command (closes: #572)
[Engine-IRC] Fixed /mode +i (on server) regression
[Engine-IRC] Fixed /mode not always printing the full mode list
[Engine-IRC] Fixed /mode printing for unjoined channels