Support ANSI MERGE

MERGE is only supported with ON KEY clause currently (similar to lookup join and index join). Since we are now supporting ANSI JOIN in Vulcan, we should also support ANSI version of MERGE, i.e., allow MERGE to have ON clause instead of ON KEY clause, such that the join between source and target of the MERGE can be done using ANSI JOIN instead of lookup join only. This will make it much easier to migrate MERGE statement from relational database to N1QL.