evaluate redundancy / error correction options

There is some danger that bitrot and storage media defects could lead to backup data loss / repository integrity issues. Deduplicating backup systems are more vulnerable to this than non-deduplicating ones, because a defect chunk affects all backup archives using this chunk.

Currently, there is a lot of error _detection_ (CRCs, hashes, HMACs) going on in borgbackup, but it has no built-in support for error _correction_ (see the FAQ about why), but it could be solved maybe using one of these approaches:
- use borg to have N (N>1) independent backup repos of your data on different targets (if N-1 targets get corrupt then, you have still 1 working left. note that there is no support to create one non-corrupt repo from 2 corrupt repos, although that might be theoretically possible for some cases).
- snapraid
- par2
- FECpp https://github.com/randombit/fecpp (BSD, C++ - make available via Cython?)
- zfec (GPL/TGPPL, Python 2.x only, PR for Python 3.x exists)
- RAID (and monitor and scrub the disks), ZFS mirror or RAIDZ* (better not use raid5 or raidz1)
- zfs copies=N option (N>1)
- specific filesystems
- ceph librados
- https://github.com/Bulat-Ziganshin/FastECC

If we can find some working approaches, we could add them to the documentation.
Help and feedback about this is welcome!