IOContext problem with multiple MapWorks cloned for multi-insert [Spark Branch]

The following query will not work:

from (select * from table0 union all select * from table1) s
insert overwrite table table3 select s.x, count(1) group by s.x
insert overwrite table table4 select s.y, count(1) group by s.y;

Currently, the plan for this query, before SplitSparkWorkResolver, looks like below:

   M1    M2
     \  / \
      U3   R5
      |
      R4

In SplitSparkWorkResolver#splitBaseWork, it assumes that the childWork is a ReduceWork, but for this case, you can see that for M2 the childWork could be UnionWork U3. Thus, the code will fail.

HIVE-9041 addressed partially addressed the problem by removing union task. However, it's still necessary to cloning M1 and M2 to support multi-insert. Because M1 and M2 can run in a single JVM, the original solution of storing a global IOContext will not work because M1 and M2 have different io contexts, both needing to be stored.