DEFAULT keyword handling in MERGE UPDATE clause issues

once HIVE-21159 lands, enable HiveConf.MERGE_SPLIT_UPDATE and run these tests.

TestMiniLlapLocalCliDriver.testCliDriver[sqlmerge_stats]
mvn test -Dtest=TestMiniLlapLocalCliDriver -Dqfile=insert_into_default_keyword.q

Merge is rewritten as a multi-insert. When Update clause has DEFAULT, it's not properly replaced with a value in the muli-insert - it's treated as a literal

INSERT INTO `default`.`acidTable`    -- update clause(insert part)
 SELECT `t`.`key`, `DEFAULT`, `t`.`value`
   WHERE `t`.`key` = `s`.`key` AND `s`.`key` > 3 AND NOT(`s`.`key` < 3)

See LOG.info("Going to reparse <" + originalQuery + "> as \n<" + rewrittenQueryStr.toString() + ">"); in hive.log

MergeSemanticAnalyzer.replaceDefaultKeywordForMerge() is only called in handleInsert but not handleUpdate(). Why does issue only show up with MERGE_SPLIT_UPDATE?

Once this is fixed, HiveConf.MERGE_SPLIT_UPDATE should be true by default