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/nodes/readfuncs.c

@@ -1412,6 +1412,7 @@ ReadCommonPlan(Plan *local_node)
 	READ_FLOAT_FIELD(total_cost);
 	READ_FLOAT_FIELD(plan_rows);
 	READ_INT_FIELD(plan_width);
+	READ_BOOL_FIELD(parallel_aware);
 	READ_INT_FIELD(plan_node_id);
 	READ_NODE_FIELD(targetlist);
 	READ_NODE_FIELD(qual);