Víctor Manuel Jáquez Leal authored Sep 30, 2020 and GStreamer Marge Bot committed Oct 04, 2020

1. Allocators don't implement memory free() methods since all the memories will
   implement dispose() returning FALSE
2. Memory/miniobject dispose() will act as memory release, enqueueing the
   release memory
3. A new allocator's method prepare_buffer() which queries the released memory
   queue and will add the requiered memories to the buffer.
4. Allocators added a GCond to synchronize dispose() and prepare_buffer()
5. A new allocator's method flush() which will free for real the memories.

While the bufferpool will

1. Remove all the memories at reset_buffer()
2. Implement acquire_buffer() calling allocator's prepare_buffer()
3. Implement flush_start() calling allocator's flush()
4. start() is disabled since it pre-allocs buffers but also calls
   our reset_buffer() which will drop the memories and later the
   buffers are ditched, something we don't want. This approach avoids
   buffer pre-allocation.

Part-of: <!1626>