Fix a small race condition when looking up the ROOT region.

There was a check-and-act sequence on the ZooKeeper object
without acquiring the appropriate lock, which means that
we could potentially cause a NullPointerException while
retrying to lookup ROOT.

Also move the ZooKeeper callback into a real class instead
of an anonymous class.
De-serialize the HBase 0.95+ znode that points to META.

This is temporarily de-serializing the znode manually, until we
get the .proto in our source tree and build them.