Only read the keyfile databases once and cache them for the remainder of
the program.

- this avoids the overhead of opening the file over and over again.

- it also avoids the data changing without us expecting it. The state
  files are internal and we don't support changing it outside of
  NetworkManager. So in the base case we read the same data over
  and over. In the worst case, we read different data but are not
  interested in handling the changes.

- only write the file when the content changes or before exiting
  (normally).

- better log what is happening.

- our state files tend to grow as we don't garbage collect old entries.
  Keeping this all in memory might be problematic. However, the right
  solution for this is that we come up with some form of garbage
  collection so that the state files are reaonsably small to begin with.