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/storage/lmgr/lwlock.c

@@ -508,6 +508,8 @@ RegisterLWLockTranches(void)
 	LWLockRegisterTranche(LWTRANCHE_LOCK_MANAGER, "lock_manager");
 	LWLockRegisterTranche(LWTRANCHE_PREDICATE_LOCK_MANAGER,
 						  "predicate_lock_manager");
+	LWLockRegisterTranche(LWTRANCHE_PARALLEL_QUERY_DSA,
+						  "parallel_query_dsa");
 
 	/* Register named tranches. */
 	for (i = 0; i < NamedLWLockTrancheRequests; i++)