MessageView: keep channel buffers alive until we actually part the chan
MessageView: keep channel buffers alive until we actually part the chan

This makes sure that we don't start receiving random private messages
(that actually belong to a closed channel - feels like a bug in
IrcBufferModel's message delivery...)

communi/communi-desktop,jpnurmi/communi-desktop,gdamjan/communi-desktop,sevanteri/communi-desktop,communi/communi-desktop