Revert recent change to RequestNamedLWLockTranche().

Commit 38b602b028 modified this function to allocate enough space
for MAX_NAMED_TRANCHES (256) requests, which is likely far more
than most clusters need.  This commit reverts that change so that
it first allocates enough space for only 16 requests and resizes
the array when necessary.  While at it, remove the check for too
many tranches from this function.  We can now rely on
InitializeLWLocks() to do that check via its calls to
LWLockNewTrancheId() for the named tranches.

Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/aLmzwC2dRbqk14y6%40nathan

src/backend/storage/lmgr/lwlock.c

@@ -610,6 +610,7 @@ void
 RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
 {
 	NamedLWLockTrancheRequest *request;
+	static int	NamedLWLockTrancheRequestsAllocated;
 
 	if (!process_shmem_requests_in_progress)
 		elog(FATAL, "cannot request additional LWLocks outside shmem_request_hook");
@@ -628,17 +629,22 @@ RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
 
 	if (NamedLWLockTrancheRequestArray == NULL)
 	{
+		NamedLWLockTrancheRequestsAllocated = 16;
 		NamedLWLockTrancheRequestArray = (NamedLWLockTrancheRequest *)
 			MemoryContextAlloc(TopMemoryContext,
-							   MAX_NAMED_TRANCHES
+							   NamedLWLockTrancheRequestsAllocated
 							   * sizeof(NamedLWLockTrancheRequest));
 	}
 
-	if (NamedLWLockTrancheRequests >= MAX_NAMED_TRANCHES)
-		ereport(ERROR,
-				(errmsg("maximum number of tranches already registered"),
-				 errdetail("No more than %d tranches may be registered.",
-						   MAX_NAMED_TRANCHES)));
+	if (NamedLWLockTrancheRequests >= NamedLWLockTrancheRequestsAllocated)
+	{
+		int			i = pg_nextpower2_32(NamedLWLockTrancheRequests + 1);
+
+		NamedLWLockTrancheRequestArray = (NamedLWLockTrancheRequest *)
+			repalloc(NamedLWLockTrancheRequestArray,
+					 i * sizeof(NamedLWLockTrancheRequest));
+		NamedLWLockTrancheRequestsAllocated = i;
+	}
 
 	request = &NamedLWLockTrancheRequestArray[NamedLWLockTrancheRequests];
 	strlcpy(request->tranche_name, tranche_name, NAMEDATALEN);