Fix NPE on Bookie server.
 This change addresses PUBSUB-118. The return value from one of the functions was not checked.

RB_ID=133174
 Fix race condition in removal of subscription channels and also clear state while releasing a topic
 - Fix race condition in removal of subscription channels and also clear state while releasing a topic

 This addresses 2 problems:
 1) PUBSUB-93 where the subscription channel manager wasn't clearing it's state when it lost a topic.
 2) Fixes a race in the Delivery manager which might affect the non-multiplexed clients. The delivery manager should know what channel it is removing a topic subscription for to guard against a race where a resubscription occurs before the delivery manager is notified of channel disconnection.

RB_ID=132585
cherry-pick BOOKKEEPER-526: multiple threads for delivery manager

it is a straightforward refactor to make the number of threads in delivery manager configurable.

RB_ID=133400