- changes to locally stored files cache:

  - store as files.<H(archive_name)>
  - user can manually control suffix via env var
  - if local files cache is not found, build from previous archive.
- enable rebuilding the files cache via loading the previous
  archive's metadata from the repo (better than starting with
  empty files cache and needing to read/chunk/hash all files).
  previous archive == same archive name, latest timestamp in repo.
- remove AdHocCache (not needed any more, slow)
- remove BORG_CACHE_IMPL, we only have one
- remove cache lock (this was blocking parallel backups to same
  repo from same machine/user).

Cache entries now have ctime AND mtime.

Note: TTL and age still needed for discarding removed files.
      But due to the separate files caches per series, the TTL
      was lowered to 2 (from 20).

…borgbackup#4915

if we detect the conditions for this (rare) race,
abort reading the file and retry.

The caller (_process_any) will do up to MAX_RETRIES
before giving up. If it gives up, a warning is logged
and the file is not written to the archive and won't
be memorized in the files cache either.

Thus, the file will be read/chunked/hashed again at
the next borg create run.