Reduce unnecessary scanner lock during read

Catalog read performed on the chunk table is getting
RowExclusiveLock while only reading the data. Reducing
to AccessShareLock will bring this in line with PG
locking convention.

src/chunk.c

@@ -3143,7 +3143,7 @@ List *
 ts_chunk_get_chunk_ids_by_hypertable_id(int32 hypertable_id)
 {
 	List *chunkids = NIL;
-	ScanIterator iterator = ts_scan_iterator_create(CHUNK, RowExclusiveLock, CurrentMemoryContext);
+	ScanIterator iterator = ts_scan_iterator_create(CHUNK, AccessShareLock, CurrentMemoryContext);
 
 	init_scan_by_hypertable_id(&iterator, hypertable_id);
 	ts_scanner_foreach(&iterator)