adaptivedemux: start/stop the manifest update loop on liveness or periodic update...

adaptivedemux: start/stop the manifest update loop on liveness or periodic update changes

Scenario:
A manifest starts out in live mode but then the recording is finalized
and a subsequent update changes the state to a non-live manifest when
the server has finished recording/transcoding/whatever with the full
list of fragments.

Without this patch, the manifest update task is never stopped on the
live->non-live transition and will busy loop, burning through one CPU
core.

https://bugzilla.gnome.org/show_bug.cgi?id=786275