Implement extended mock persistence commands

This adds mock commands to manipulate individual keys within a
particular storage domain.

Change-Id: I1083b68877249841a82b97b3c5f30d4f8b70bbb5
Reviewed-on: http://review.couchbase.org/27715
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>