Enable operations to complete out of order on a single application conneciton

Problem:

Currently we have a head of line problem in servicing requests on a single application connection. If the first operation is a long running one, the following ones might be short operations but will have to wait to return. This is a common problem - typically each client has one connection to the server and will multiplex all its requests on one connection to memcached - causing short requests to suffer.

As a result of current behavior applications see more timeouts than expected on short requests.

Suggested Improvement:

The improvement is to allow out of order responses to the application. Allowing requests to return as they finish will improve response time for the short operations. 

Caveat: With this change, behavior of requests without responses (set operations) will change - is this a backward compatibility concern?

Dave Rigby, David Haikney please enhance/edit description as appropriate.