Accept reasonable plan changes in select_parallel

Accept reasonable plan changes in select_parallel

All the accepted plan changes are simply adding Remote Subquery, and
seem correct and reasonable. Where possible, I've verified that the
older XL versions produce the same (or very similar) plan.

There are also three additional minor fixes:

1) An extra EXPLAIN query, as EXPLAIN ANALYZE hides the part below
   Remote Subquery, making it mostly useless. The extra EXPLAIN
   shows the whole plan and addresses this.

2) Postgres-XL does not support subtransactions, so the block setting
   effective_io_concurrency was failing, and aborting the surrounding
   transaction. Removing the EXCEPTION clause may cause issues on
   systems not supporting this GUC, but that should be rare.

3) Removed a section of expected output, matching a block removed
   from the SQL script.