Protect pbs with proper (aka locked) refcounting.

The PlaybackSock must not be immediatly deleted on connectionClosed()
if some other requests are still using it. This leads to segfaults in
the MBE when the SBE disconnects (#11306).
(cherry picked from commit 7b305848b0584bcebc55d6fe5a670ef7d2f55360)

Refs #11306
Signed-off-by: Stuart Auchterlonie <stuarta@squashedfrog.net>