bufferpool: avoid excessive GstPoll activity

bufferpool: avoid excessive GstPoll activity

Keep an extra write ref on the control socket. This ensures that we
avoid a read/write on the socket when going from non-empty->empty->not-empty.
We remove the write ref only when we actually are empty and we need to
wait for flushing or a new buffer.

This makes the bufferpool benchmark about 30% faster than the pure
malloc implementation.