OAK-1055: Occasional test failure in ObservationTest.observation
OAK-1060: Periodically poll for external events
- Remove PostCommitHook argument from NodeStore.merge and make ChangeDispatcher part of NodeStore implementations that implement Observable (i.e. support observation).
- Use :commit-info on the root node to pass commit information across

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1532782 13f79535-47bb-0310-9956-ffa450edef68