rcompress: do a repo-wide (re)compression

[ThomasWaldmann]

just reads all chunks and recompresses them if they are not already using the desired compression type and level.

some improvements left to do:

• if the usual DecidingCompressor (e.g. for zstd) we use chooses to return NONE or LZ4 compressed data (because that was shorter than the desired zstd), running the same rcompress command again, will try again to compress these chunks (with the same non-zstd result).

note: this can't be backported to borg 1.x because it uses some of the borg2-only features (like not having csize in archive metadata or elsewhere, like fetching the separate metadata only to check for the current compression type and level).

[codecov-commenter]

Codecov Report

Merging #7037 (78b1301) into master (78b1301) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 78b1301 differs from pull request most recent head aaf4d18. Consider uploading reports for the commit aaf4d18 to get more accurate results

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  Misses       1347     1347           
  Partials      536      536           

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