WIP Add par2 parity files and auto repair to backends

[BlueBlock]

(I'm putting this out to get early feedback. This top-post will be updated with all progress.)

Still is a work-in-progress

Summary:

This PR adds support for adding parity files to all files uploaded to a backend.

One option has been added to control the use of parity files.

• parity-file-redundancy: represents the percentage of par2 redundancy (value 0 through 100, a default of 0, a value of 0 disables use of parity files)

Additional discussion on the topic can be found here.

To be done:

• need solution for adding par2cmdline functionality to linux (manual install by user?)

Features:

• all backends are supported
• (Windows) par2cmdline added to provide par2 functionality
• a backend file has an individual parity file, a 1:1 relationship
• a parity file is named after the source backend filename + ".par2.zip"
• a parity file are generated and accompanies the upload of the backend file
• a parity file removed when the source backend file is removed
• an automatic repair attempt will be made when source backend file is damaged
• a repaired source backend file will be re-uploaded and replace the corrupted source backend file
• parity files can be enabled on existing backups and parity files uploaded to backend (TBD)

[duplicatibot]

This pull request has been mentioned on Duplicati. There might be relevant details there:

https://forum.duplicati.com/t/failed-invalid-header-marker/2871/44

[warwickmm]

Closing this due to lack of activity. We can revisit this later if necessary.

[duplicatibot]

This pull request has been mentioned on Duplicati. There might be relevant details there:

https://forum.duplicati.com/t/reliability-of-duplicati-2-0/10435/6

[duplicatibot]

This pull request has been mentioned on Duplicati. There might be relevant details there:

https://forum.duplicati.com/t/btrfs-as-a-filesystem-for-the-backup-disc/12614/2