Fix MemcachedMock to align with latest SPY changes.

Change-Id: I113802e89d1f74f93859e7298f06a693ea402d6b
Reviewed-on: http://review.couchbase.org/27261
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>