Regularly store real time and real time offset

[PatrickKa]

Description

Every 10 s or something, the real time offset is read from the persistent state and published. After that, the current real time is saved to the persistent state. This tries to mitigate the loss of real time information after a reset of the COBC. What makes all of this tricky is that the function for updating the real time estimation also wants to change the time offset (see #223). A topic can, however, only have a single publisher. Maybe there is some other Rodos construct that is better suited for this.

Edit: As I already wrote in #199, I don't think that we need a topic or a separate thread. We just need to make sure that the real time offset and the estimated real time are stored regularly as persistent variables. This could be done in the telemetry thread, which needs to touch all persistent variables anyway.