Various cleanups and warning fixes.

Particularly interesting is the change to ZooKeeperUpdatingPersistentDirectory.
I have no idea if that ever worked, as if it ever removed a node, it would get
a ConcurrentModificationException.

In JobValidator, the `if (jobId == null)` could never be reached as an NPE would
be thrown before it got there.

As usual, kudos to the writers of Eclipse for catching most of these.  The ZKUPD
issue I happened to notice when doing the release build/test.