Add client tests

These tests cover the basic memcached functionality of the Mock. They
also cover the new extended mock commands for persistence and key state

Change-Id: Ia648436ac8ebc8b921687e27093b4b7bc77630a5
Reviewed-on: http://review.couchbase.org/27716
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>