this is intended to be a generic helper to (de)serialize job states
(e.g., sync, verify, and so on)
writes a json file into '/var/lib/proxmox-backup/jobstates/TYPE-ID.json'
the api creates the directory with the correct permissions, like
the rrd directory
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>