Sanity check data in RocksDB on startup

[holtgrewe]

Is your feature request related to a problem? Please describe.
Right now, the programs using RocksDB databases with protobuf data will fail if the protobuf data is invalid (e.g., outdated) when the first query returns data. This is a bit late.

Describe the solution you'd like
We should check on startup whether the data behind the first key has valid protobuf.

Describe alternatives you've considered
N/A

Additional context
Possibly we want to have this functionality in annonars and just call it here.