Implement support for MERGE command in ItemCreateJob

By setting ItemCreateJob::setMergeIfExists to true, the server job will
use MERGE instead of AK-APPEND command. Server will try to merge the
new item with an existing one (if any such exists) by GID and RID match.

If no such item exists, the server will fallback to AK-APPEND behavior and
a new item is inserted. Otherwise the merged item is returned.