A value given in config overrides values read from the persistent data file.
To let the loaded parameter have precedence, configure a default only.
The write_<param> method of a persistent parameter is now always called
on startup.
- add tests for persistent behaviour
+ simplify Modules.writeInitParams: remove started_callback argument
Change-Id: I08b49de52e9d9a2ed0918018eb2fe538141a4f5e
