pbs-datastore: use ConfigVersionCache for datastore

pbs-datastore: use ConfigVersionCache for datastore

instead of relying on the content of some configs

previously, we always read and parsed the config file, and only
generated a new config object when the path or the 'verify-new' option
changed.

now, we increase the datastore generation on config save, and if that
changed (or the last load is 1 minute in the past), we always
generate a new object

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>