Describe correct procedure for backups and test it

[giovannipizzi]

In particular, if rsync is used during packing, check if there is the risk of having corrupted data (e.g. if the pack files are transferred before the index and this then refers to missing data, if the loose files are not transferred but the packs have been already transferred, etc.)

[giovannipizzi]

This is very important - note that there is an official procedure also to backup SQLite files (see here), probably one cannot just run rsync on them.

Note that the standard python3 sqlite library has a method to perform backups: https://docs.python.org/3/library/sqlite3.html#sqlite3.Connection.backup

[giovannipizzi]

@eimrek is working on the docs for AiiDA backups. Good to back port something in this repo, or at least refer to those (AiiDA) docs from the disk-objectstore docs.

[eimrek]

Thanks. Possibly makes sense to move the dos backup function to this repo https://github.com/aiidateam/aiida-core/blob/backup-script/aiida/storage/psql_dos/backend.py#L429

[eimrek]

done in #161