Added retry policy allowing fallback from LOCAL_QUORUM to QUORUM initial...

This retry policy wraps the DowngradingConsistencyRetryPolicy, essentially applying the same logic, EXCEPT when the initial consistency level is set to LOCAL_QUORUM. In that case it first attempts at QUORUM, then falls back to ONE. 

The idea is that you can make calls at LOCAL_QUORUM, which will fall back to QUORUM first if needed. The cost of full QUORUM is preferable in some use cases to ONE, where you want to be really sure a write made it. This is especially likely if RF=3 and you've lost two of your local replicas (resulting in the failed LOCAL_QUORUM).  Then if all else fails it goes to ONE.