OAK-959: Deadlock between ChangeDispatcher and SegmentNodeStoreService
Use explicit lock in ChangeDispatcher instead of object monitor. Try to acquire lock for getting external changes and do nothing if this fails to avoid dead locking with incoming changes.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1518226 13f79535-47bb-0310-9956-ffa450edef68
OAK-960 Enable session refresh state coordination between multiple session in single thread
Reduce warnings about long living sessions to sessions that do not refresh

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1518227 13f79535-47bb-0310-9956-ffa450edef68