Provide a DSA area for all parallel queries.

This will allow future parallel query code to dynamically allocate
storage shared by all participants.

Thomas Munro, with assorted changes by me.

src/include/storage/lwlock.h

@@ -210,6 +210,7 @@ typedef enum BuiltinTrancheIds
 	LWTRANCHE_BUFFER_MAPPING,
 	LWTRANCHE_LOCK_MANAGER,
 	LWTRANCHE_PREDICATE_LOCK_MANAGER,
+	LWTRANCHE_PARALLEL_QUERY_DSA,
 	LWTRANCHE_FIRST_USER_DEFINED
 }	BuiltinTrancheIds;