Fix the hanging OpenTSDB bug after a restart of HBase region servers.

The bug existed with HBase versions lower than 0.95. The bug was that
ProtocolVersionCB of RegionClient did not properly handle exceptions like
ServerNotRunningYetException during a HBase restart. The side effect was that
neither the becomeReady nor the cleanup method of the corresponding region
client was called because the channel was already connected and healthy. As a
result, any attempt of sending requests to the region server via sendRpc ended
up accumulating the requests to the pending_rpcs. The fix was closing the
corresponding channel when ProtocolVersionCB gets an exception. Also more
loggings were added that were useful for debugging this bug.