OAK-1292: Avoid async indexer commits when there are no changes to index

Make the callback argument @Nonnull to avoid repeated null checks

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1559840 13f79535-47bb-0310-9956-ffa450edef68
OAK-1292: Avoid async indexer commits when there are no changes to index

Merge the two-level conditional in AsyncIndexUpdate.run() to a single if.
Replace the final AtomicBoolean flag with a member variable of an internal AsyncUpdateCallback object.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1559841 13f79535-47bb-0310-9956-ffa450edef68