bufferpool: Add method and virtuals to set flushing state

bufferpool: Add method and virtuals to set flushing state

Currently there is no other way to unlock a buffer pool other then
stopping it. This may have the effect of freeing all the buffers,
which is too heavy for a seek. This patch add a method to enter and
leave flushing state. As a convenience, flush_start/flush_stop
virtual are added so pool implementation can also unblock their own
internal poll atomically with the rest of the pool.  This is fully
backward compatible with doing stop/start to actually flush the pool
(as being done in GstBaseSrc).

https://bugzilla.gnome.org/show_bug.cgi?id=727611