Add support for multiple WHEN MATCHED clauses

Different vendors have different rules.
HIVE-14943 only allows 1 WHEN MATCHED clause

IBM Allows > 1 WHEN MATCHED clause https://www.ibm.com/support/knowledgecenter/SSEPGG_9.5.0/com.ibm.db2.luw.sql.ref.doc/doc/r0010873.html

So does SQL Server https://msdn.microsoft.com/en-us/library/bb510625.aspx

everyone seems to support update/delete/insert - in that order