Power outage restoration

[xtrojak]

In the case of power outage, it would be great if PBRcontrol could be restored to the previous state. This includes two steps:

1. start PBRcontrol API on system startup
2. restore all running experiments

Step 2 is quite tricky, it requires to add more state logging to DB such it is possible to recognize whether an experiment was running before PBRcontrol was terminated and also which node with particular settings were running.

Relevant to issue #11.