Fix mistaken failure to allow parallelism in corner case.

If we try to run a parallel plan in serial mode because, for example,
it's going to be scanned via a cursor, but for some reason we're
already in parallel mode (for example because an outer query is
running in parallel), we'd incorrectly try to launch workers.
Fix by adding a flag to the EState, so that we can be certain that
ExecutePlan() and ExecGather()/ExecGatherMerge() will have the same
idea about whether we are executing serially or in parallel.

Report and fix by Amit Kapila with help from Kuntal Ghosh.  A few
tweaks by me.

Discussion: http://postgr.es/m/CAA4eK1+_BuZrmVCeua5Eqnm4Co9DAXdM5HPAOE2J19ePbR912Q@mail.gmail.com

src/backend/executor/execMain.c

@@ -1702,6 +1702,7 @@ ExecutePlan(EState *estate,
 	if (!execute_once)
 		use_parallel_mode = false;
 
+	estate->es_use_parallel_mode = use_parallel_mode;
 	if (use_parallel_mode)
 		EnterParallelMode();