MockModuleBuilder: manage timestamps on mock config files
    
    The mod time for the mock configuration file is used to determine
    whether the root cache is out-of-date or not, so we want to avoid
    changing the configuration timestamps when we don't change content
    when we're just writing a per-thread mock configuration file again
    with no substantive changes.
    
    We do this by only updating the master mock.cfg file when we're
    actually adding content, and propagating its mod time to the
    per-thread configuration files.