Implement Upr Set VBucket State API

The tap and upr set vbucket state commands are very similar. See the current implementation for tap here:

https://github.com/membase/ep-engine/blob/master/src/ep_engine.cc#L2552

We want to keep the tap parsing here, but move the rest of the logic to the ConnHandler API which is here.

https://github.com/membase/ep-engine/blob/master/src/tapconnection.h#L387

I have added an API in the UprConsumer to implement this API which current returns ENGINE_ENOTSUP. The best thing to do is probably to move this API into the Consumer class and then have both tap and upr consumers call it.

https://github.com/membase/ep-engine/blob/master/src/tapconnection.h#L746
https://github.com/membase/ep-engine/blob/master/src/upr-consumer.h#L71