CBQE-1191: fix for multi-bucket postconditions

this patch remedies a problem where postconditions
were not reached for serveral reasons.
1) failing to retrieve a workload object from the cache
2) the postcondition_handler method being overwritten when
   ops_builder flag was set.

Change-Id: I8d9549c74ed643766365187bb7a3a76baa19dc25
Reviewed-on: http://review.couchbase.org/25471
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>