Add 'CommandStatus' class for mock handlers

Change-Id: I18655b09efba34691d0802fc6320bd14ae0dddcb
Reviewed-on: http://review.couchbase.org/29988
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>