adaptivedemux: Don't hold locks when pushing FLUSH_START

adaptivedemux: Don't hold locks when pushing FLUSH_START

Some actions (Qos, reconfigure, ...) might take place before we finish pushing out flush_start.

One problem would be that:
1) The QOS handling in adaptivedemux takes the MANIFEST LOCK
  That QOS event comes from basesink with its PREROLL_LOCK taken
2) FLUSH_START is sent from adaptivedemux with the MANIFEST_LOCK taken and the basesink flushing handler needs to take the PREROLL_LOCK

 => deadlock

https://bugzilla.gnome.org/show_bug.cgi?id=781320