multiqueue: only block serialized query when it's safe

multiqueue: only block serialized query when it's safe

We must be certain that we don't cause a deadlock when blocking the serialized
queries. One such deadlock can happen when we are buffering and downstream is
blocked in preroll and a serialized query arrives. Downstream will not unblock
(and allow our query to execute) until we complete buffering and buffering will
not complete until we can answer the query..

https://bugzilla.gnome.org/show_bug.cgi?id=702840