Client Durability support.

You get a NotSupportedError attempting to do a durable write on a 6.0 or earlier cluster. At least, when you use a DurabilityOptionBlock, in the scenarios_t.py tests. But, that used to be supported, just differently.

We should maybe see if

• we can be backwards-compatible (if LCB does somehow with maybe the old persistTo, replicateTo stuff?
• we get bucketCapabilities when we open a bucket, and check before attempting the durable write. Perhaps logging something helpful or even raising a different exception?
• maybe we can clever and convert (if there is a way) so the call is successful on a 6.0 cluster?

I didn't have time to look into it while fixing the integration tests, so I skip durability on < 6.5 and this feels incomplete. So, lets look harder and make it better if possible.