Handle UNION pushdown edge case that triggers slow table mode processing

Sometimes queries runs slow , but suddenly runs fast without "order by" clause
or wrapping into a subselect like select * from (<select query>)
Even every case must be fixed , it should be a possibility to force the behaviour
of wrapping the query in a subselect, so application must not be changed,
so something like
columnstore_forced_subselect,
so simply set

set columnstore_forced_subselect = 1;

can work as a workarround,
Must be certainly dynamically and possible with global and session level.

Description:
Add an extra step to tuple union step. This will enable processing of unions better. Thus handling it on the columnstore side. Enable order by after unions, enabling sorting. See leonid.fedorov for details