aggregator: Replace GMainContext with GAsyncQueue (v2)
aggregator: Replace GMainContext with GAsyncQueue (v2)

The previous implementation kept accumulating GSources,
slowing down the iteration and leaking memory.

Instead of trying to fix the main context flushing, replace
it with a GAsyncQueue which is simple to flush and has
less overhead.

https://bugzilla.gnome.org/show_bug.cgi?id=736782

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer