Track file trailer only in debug builds

The PR timescale#5410 changes the handling of the file_trailer_received flag. It
is now only used in asserts and not in any other kind of logic. This
patch encapsulates the file_trailer_received in a USE_ASSERT_CHECKING
macro.

tsl/src/remote/copy_fetcher.c

@@ -19,8 +19,10 @@ typedef struct CopyFetcher
 	/* Data for virtual tuples of the current retrieved batch. */
 	Datum *batch_values;
 	bool *batch_nulls;
-	bool file_trailer_received;
 	AsyncRequest *req;
+#ifdef USE_ASSERT_CHECKING
+	bool file_trailer_received;
+#endif
 } CopyFetcher;
 
 static void copy_fetcher_send_fetch_request(DataFetcher *df);
@@ -61,7 +63,10 @@ static void
 copy_fetcher_reset(CopyFetcher *fetcher)
 {
 	fetcher->state.open = false;
+
+#ifdef USE_ASSERT_CHECKING
 	fetcher->file_trailer_received = false;
+#endif
 
 	if (fetcher->req != NULL)
 	{
@@ -449,7 +454,9 @@ copy_fetcher_complete(CopyFetcher *fetcher)
 				 * nor the expected number of columns. This provides an extra
 				 * check against somehow getting out of sync with the data.
 				 */
+#ifdef USE_ASSERT_CHECKING
 				fetcher->file_trailer_received = true;
+#endif
 
 				/* Next PQgetCopyData() should return -1, indicating EOF and
 				 * that the remote side ended the copy. The final result