Retain element ordering in entity callbacks.

We now use concatMap on result Fluxes to retain the object order. Previously, we used flatMap on a flux that has led to changes in element ordering.

Closes #777

Author: mp911de

src/main/java/org/springframework/data/r2dbc/core/R2dbcEntityTemplate.java

@@ -409,7 +409,7 @@ <T, P extends Publisher<T>> P doSelect(Query query, Class<?> entityClass, SqlIde
 			return (P) ((Mono<?>) result).flatMap(it -> maybeCallAfterConvert(it, tableName));
 		}
 
-		return (P) ((Flux<?>) result).flatMap(it -> maybeCallAfterConvert(it, tableName));
+		return (P) ((Flux<?>) result).concatMap(it -> maybeCallAfterConvert(it, tableName));
 	}
 
 	private <T> RowsFetchSpec<T> doSelect(Query query, Class<?> entityClass, SqlIdentifier tableName,
@@ -1114,7 +1114,7 @@ public Mono<T> first() {
 
 		@Override
 		public Flux<T> all() {
-			return delegate.all().flatMap(it -> maybeCallAfterConvert(it, tableName));
+			return delegate.all().concatMap(it -> maybeCallAfterConvert(it, tableName));
 		}
 	}