aggregator: add min-upstream-latency property.
aggregator: add min-upstream-latency property.

This is exposed as a solution to the use case of plugging in
sources with a higher latency after the aggregator has started
playing with an initial set of sources, allowing to avoid resyncing.

https://bugzilla.gnome.org/show_bug.cgi?id=797213

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