Push PARTIAL TopN into TableScan

[findepi]

PushTopNIntoTableScan was originally added in the optimizer which creates PARTIAL TopN nodes and pushes them through UNION and outer join.

#6847 simplifies and fixes some things, but for this it moves the PushTopNIntoTableScan into earlier phase (where other PushXxxIntoTableScan live) and disables it for PARTIAL TopN nodes. The intent is – if TopN can be fully pushed down, it should be pushed down at once, before it gets split into PARTIAL and FINAL.

We should revisit this – maybe we should push the SINGLE TopN where possible, but later also attempt to push PARTIAL TopN once these get created.

Add appropriate testing.