Small ALTER affecting GI fixes

Two bug cases, caught by FTS, in refactor from #1020:
- Table in the group but *not* participating in a GI being rebuilt
- ALTER causing both a GI to be dropped and another GI to be rebuilt

The former hit an assertion due the writeRow() getting passes the GI to be maintained when that table didn't participate. Fix by tightening what is including the in `groupIndexes` list (as MMcM hinted at in the review).

The latter resulted in an AIS validation failure due to a stale table definition being copied in the new drop/drop/alter/recreate flow. Make sure and use the latest definition of the table