Fix possible crash reading pg_stat_activity.

With the old code, a backend that read pg_stat_activity without ever
having executed a parallel query might see a backend in the midst of
executing one waiting on a DSA LWLock, resulting in a crash.  The
solution is for backends to register the tranche at startup time, not
the first time a parallel query is executed.

Report by Andreas Seltenreich.  Patch by me, reviewed by Thomas Munro.

src/backend/executor/execParallel.c

@@ -486,7 +486,6 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
 		shm_toc_insert(pcxt->toc, PARALLEL_KEY_DSA, area_space);
 		pei->area = dsa_create_in_place(area_space, dsa_minsize,
 										LWTRANCHE_PARALLEL_QUERY_DSA,
-										"parallel_query_dsa",
 										pcxt->seg);
 	}