Use Curator leader latch instead of checking the order of Oozie servers

We currently have a few tasks (e.g. Purging old jobs) that we only want to do in one Oozie server. We currently simply check which Oozie server is first in ZooKeeper's list of servers (i.e. the order they connected). We haven't seen any problems with this, but it might be a good idea to replace this with Curator's leader-latch, which sounds more robust.

The leader path should probably be something like "/services/leader".

Make sure errors and edge cases are handled properly, including what happens when the leader dies without unregistering, etc.

http://curator.apache.org/curator-recipes/leader-latch.html