MXS-1506: Combine housekeeper task types

The tasks themselves now control whether they are executed again. To
compare it to the old system, oneshot tasks now return `false` and
repeating tasks return `true`.

Letting the housekeeper remove the tasks makes the code simpler and
removes the possibility of the task being removed while it is being
executed. It does introduce a deadlock possibility if a housekeeper
function is called inside a housekeeper task.

server/modules/routing/binlogrouter/blr.h

@@ -1018,7 +1018,7 @@ extern const char *blr_skip_leading_sql_comments(const char *);
 extern bool blr_fetch_mariadb_gtid(ROUTER_SLAVE *,
                                    const char *,
                                    MARIADB_GTID_INFO *);
-extern void blr_start_master_in_main(void* data);
+extern bool blr_start_master_in_main(void* data);
 extern bool blr_binlog_file_exists(ROUTER_INSTANCE *router,
                                    const MARIADB_GTID_INFO *info_file);