Implement persistency option for application output variables

[killenb]

DeviceToControlSystem variables can conceptually not be presisted by the control system. The application has to take care of this in case the values cannot be re-calculated when re-starting the server.

Task: The Output should get an option to persist this variable (off by default). When enabled, the variable is always written to file when a write is called. When the application is started/initialised, the variable content is read from the file.

Details:

• The implementation should happen with a decorator. Like this any output (to Device, Control System or to other ApplicationModules) can be saved and restored.
• Make sure to flush the file after each write. Leave a comment in the code that this has to be tested manually by killing the application with -9 and check that the last value is restored correctly at server start. This kind of test is not well suited for an automated test.

[mhier]

I just hit another use case for this. My workaround now was to use an input with return channel instead and ignore any values written to that input. This could be another way of implementing this feature (of course while avoiding the quirks I have to use now), as it would allow to use the same persistency layer as for the input variables (i.e. the one from the control system).

[mhier]

Now on redmine #8362