Fix multi-insert query and write qtests

On HIVE-16415, it was found that the bug reported to be fixed in HIVE-14519 was not actually fixed.

This task is to find the problem, fix it, and add qtests to verify no future regression.

Specifically, the following query does not produce correct answers:

From (select * from src) a
insert overwrite directory '/tmp/emp/dir1/'
select key, value
insert overwrite directory '/tmp/emp/dir2/'
select 'header'
limit 0
insert overwrite directory '/tmp/emp/dir3/'
select key, value 
where key = 100;

This gives incorrect result in master. All dirs end up with 0 rows instead of just dir2.