Increase MAX_SYSCACHE_CALLBACKS from 20 to 32.

By my count there are 18 callers of CacheRegisterSyscacheCallback in the
core code in HEAD, so we are potentially leaving as few as 2 slots for any
add-on code to use (though possibly not all these callers would actually
activate in any particular session).  That doesn't seem like a lot of
headroom, so let's pump it up a little.

src/backend/utils/cache/inval.c

@@ -174,7 +174,7 @@ static int	maxSharedInvalidMessagesArray;
  * assumes there won't be very many of these at once; could improve if needed.
  */
 
-#define MAX_SYSCACHE_CALLBACKS 20
+#define MAX_SYSCACHE_CALLBACKS 32
 #define MAX_RELCACHE_CALLBACKS 5
 
 static struct SYSCACHECALLBACK