CBQE-0: fixed merge_keys to not use valid or deleted data members from Partition class

Change-Id: I303c855fd7bd5c670fb7cd1b687923a2281f9762
Reviewed-on: http://review.couchbase.org/33791
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>