Allow a parallel context to relaunch workers.

This may allow some callers to avoid the overhead involved in tearing
down a parallel context and then setting up a new one, which means
releasing the DSM and then allocating and populating a new one.  I
suspect we'll want to revise the Gather node to make use of this new
capability, but even if not it may be useful elsewhere and requires
very little additional code.

src/include/access/parallel.h

@@ -35,6 +35,7 @@ typedef struct ParallelContext
 	dlist_node	node;
 	SubTransactionId subid;
 	int			nworkers;
+	int			nworkers_launched;
 	parallel_worker_main_type entrypoint;
 	char	   *library_name;
 	char	   *function_name;