Do not create partitions that are not to be retained

I realized this requires tests and possibly other changes, but it's just a start to make the proposal from the pgpartman#649 more concrete.
I did try this and works for my use case.

sql/functions/run_maintenance.sql

@@ -211,6 +211,10 @@ LOOP
         SELECT child_start_time INTO v_last_partition_timestamp
             FROM @[email protected]_partition_info(v_parent_schema||'.'||v_last_partition, v_row.partition_interval, v_row.parent_table);
 
+        IF v_row.retention IS NOT NULL THEN
+            v_last_partition_timestamp := greatest(v_last_partition_timestamp, CURRENT_TIMESTAMP - v_row.retention::interval);
+        END IF;
+
         -- Must be reset to null otherwise if the next partition set in the loop is empty, the previous partition set's value could be used
         v_current_partition_timestamp := NULL;