Implement the upr flush api on the producer and consumer

Consumer Side:

The first ting that we need to do is to implement flush on the consumer side. This should also be much easier than the producer side implementation. To do this we need to fill out the function below.

https://github.com/membase/ep-engine/blob/master/src/upr-consumer.cc#L154

You will need to check to check that the opaque for the stream is valid and then you can call the reset function which is defined here.

https://github.com/membase/ep-engine/blob/master/src/ep.cc#L2211

This should be pretty straightforward and I think we can just add a single unit test to make sure that all of the items are actually removed from the vbucket.

Producer Side:

(Will add soon)