webrtcsink: wait for Sessions to end

State::finalize_session() asynchronously sets the Session pipeline to Null. In some cases, sessions webrtcbin could terminate their transition to Null after webrtcsink had reached Null.

This commit adds a set of finalizing_sessions. When the finalization process starts, the session is added to the set. After webrtcbin has reached the Null state, the session is removed from the set and a condvar is notified.

In unprepare, webrtcsink loops until the finalizing_sessions set is empty, awaiting for the condvar to be notified when it's not.