[Backport MB-23733] - Port union scan enhancements to 4.6.4

Currently the plan for UNION SCAN is correctly generated in 5.0.

Considering that most enterprise customers are probably not going to get to 5.X for atleast another 12 months it would be worthwhile to port back the UNION SCAN enhancement in to 4.6.X

It currently works correctly for the following scenario

k0="X" OR k1="Y"

It does not work for the following

(k0="X" OR k1="Y") AND k3="Z"

Union scan does provide significant benefits on OR queries so that existing indexes can be used without having to do a UNION ALL i.e. manually breaking up the query to make it optimal.