Kfsio library: implement optional crypto keys persistence by creating a thread that writes / updates all currently valid crypto keys plus newly generated key, before a newly generated key becomes current. Load crypto keys from the specified file on initial set parameters and start the crypto keys write thread if crypto keys file name parameter is not empty. Fix crypto keys write order by writing the current and pending current, if valid, last. Fix assertion condition reversal in the keys write method. Ensure that the current key is valid once initial parameters load completes, unless the initial parameter load returns an error.