Re-synchronize chunks cache with repository.
If present, uses a compressed tar archive of known backup archive
indices, so it only needs to fetch infos from repo and build a chunk
index once per backup archive.
If out of sync, the tar gets rebuilt from known + fetched chunk infos,
so it has complete and current information about all backup archives.
Finally, it builds the master chunks index by merging all indices from
the tar.
Note: compression (esp. xz) is very effective in keeping the tar
relatively small compared to the files it contains.
Use python >= 3.3 to get better compression with xz,
there's a fallback to bz2 or gz when xz is not supported.