Rename force_parallel_mode to debug_parallel_query

force_parallel_mode is meant to be used to allow us to exercise the
parallel query infrastructure to ensure that it's working as we expect.
It seems some users think this GUC is for forcing the query planner into
picking a parallel plan regardless of the costs.  A quick look at the
documentation would have made them realize that they were wrong, but the
GUC is likely too conveniently named which, evidently, seems to often
result in users expecting that it forces the planner into usefully
parallelizing queries.

Here we rename the GUC to something which casual users are less likely to
mistakenly think is what they need to make their query run more quickly.

For now, the old name can still be used.  We'll revisit if the old name
mapping can be removed once the buildfarm configs are all updated.

Reviewed-by: John Naylor
Discussion: https://postgr.es/m/CAApHDvrsOi92_uA7PEaHZMH-S4Xv+MGhQWA+GrP8b1kjpS1HjQ@mail.gmail.com

src/backend/commands/explain.c

@@ -756,8 +756,8 @@ ExplainPrintPlan(ExplainState *es, QueryDesc *queryDesc)
 	/*
 	 * Sometimes we mark a Gather node as "invisible", which means that it's
 	 * not to be displayed in EXPLAIN output.  The purpose of this is to allow
-	 * running regression tests with force_parallel_mode=regress to get the
-	 * same results as running the same tests with force_parallel_mode=off.
+	 * running regression tests with debug_parallel_query=regress to get the
+	 * same results as running the same tests with debug_parallel_query=off.
 	 * Such marking is currently only supported on a Gather at the top of the
 	 * plan.  We skip that node, and we must also hide per-worker detail data
 	 * further down in the plan tree.