repository format conversion for existing repos

[ThomasWaldmann]

related to #1704:

borg < 1.3 repos will have all data in PUT entries (not: PUT2 as generated by #1704 solution).

while we could initially offer PUT2 only for new repos / newly stored data, we should also provide a conversion from PUT to PUT2 so that

• we later can remove the PUT based code in borg.repositorycompletely.
• we can have the better integrity checks of PUT2 (crc32 only over header data, cryptographic hash for content data)
• we can have the better performance of PUT2 (not having to read all data to verify the crc32, if we are only interested in the header)

Conversion is quite easy, but what's harder is how to best deal with very large repositories, potential interruptions of the process, etc.

[ThomasWaldmann]

Options:

• Repository.check + LoggedIO.recover_segment
• Repository.compact_segments
• if it was optimized better: just borg recreate all archives
• separate / new code (use Repository.scan, ...)

[ThomasWaldmann]

Currently planned to do this via borg transfer, see #6663.

[ThomasWaldmann]

done via borg transfer, see borg2 branch, 2.0.0a2 release.