Fix datasource GC reference_count

It is incremented from `@subscribed_channels` (`datasource.coffee:999`) and decremented from `channel_mapping` (`datasource.coffee:1014`).

__It needs to be consistent__. I choose `channel_mapping` because it's more loose and you can hold a channel's reference to send it to a widget at any point in time without worrying wether it still is alive or not.