Fix a bug where StreamingClient will throw NPE if not initialized

The StreamingClient.commitVoldemort() method would throw NPE if
it is called before "initStreamingSessions" is called. The fix
will make it skip proper operations so the commit() method will
not throw NPE
Keep consistent preferred reads and writes config for zone clipping
Modified RO Rebalance Test to cover one more case

Cover the case that a partition as secondary replica become primary
partition