Make sequential scans parallel-aware.

In addition, this path fills in a number of missing bits and pieces in
the parallel infrastructure.  Paths and plans now have a parallel_aware
flag indicating whether whatever parallel-aware logic they have should
be engaged.  It is believed that we will need this flag for a number of
path/plan types, not just sequential scans, which is why the flag is
generic rather than part of the SeqScan structures specifically.
Also, execParallel.c now gives parallel nodes a chance to initialize
their PlanState nodes from the DSM during parallel worker startup.

Amit Kapila, with a fair amount of adjustment by me.  Review of previous
patch versions by Haribabu Kommi and others.

src/backend/executor/execAmi.c

@@ -439,6 +439,15 @@ ExecSupportsBackwardScan(Plan *node)
 	if (node == NULL)
 		return false;
 
+	/*
+	 * Parallel-aware nodes return a subset of the tuples in each worker,
+	 * and in general we can't expect to have enough bookkeeping state to
+	 * know which ones we returned in this worker as opposed to some other
+	 * worker.
+	 */
+	if (node->parallel_aware)
+		return false;
+
 	switch (nodeTag(node))
 	{
 		case T_Result: