- 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.

we discard all files cache entries referring to files
with timestamps AFTER we started the backup.

so, even in case we would back up an inconsistent file
that has been changed while we backed it up, we would
not have a files cache entry for it and would fully
read/chunk/hash it again in next backup.