[FELIX-4670] Deadlock in Felix HTTP service

This commit removes the locking in the HandlerRegistry and replaces it with the use of ConcurrentMaps. The HandlerRegistry lock is one of the two locks involved in the deadlock described in FELIX-4670.
Unit tests included.


git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1632257 13f79535-47bb-0310-9956-ffa450edef68