aggregator: Consistently lock some members
aggregator: Consistently lock some members

Some members sometimes used atomic access, sometimes where not locked at
all. Instead consistently use a mutex to protect them, also document
that.

https://bugzilla.gnome.org/show_bug.cgi?id=742684

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