Persisted configurations aren't backwards compatible

The port parameter that has a default value of 3306 in 2.3 does not have a default value in 2.2 which prevents downgrades when persisted configurations have been created.

This is caused by the conditional dumping code in dump_if_changed that only persists the change if it is different from the default value. This was intended as a way to allow new user-defined configuration parameters in the main configuration file but it also causes this problem.