speed up chunks cache sync, fixes #18
speed up chunks cache sync, fixes #18

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.

raxenak/borg,ionelmc/borg,level323/borg,edgewood/borg,RonnyPfannschmidt/borg,edgimar/borg,ionelmc/borg,raxenak/borg,edgewood/borg,mhubig/borg,level323/borg,edgimar/borg,raxenak/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,edgewood/borg,edgimar/borg,level323/borg,ionelmc/borg,edgewood/borg,mhubig/borg,RonnyPfannschmidt/borg,edgimar/borg,mhubig/borg,RonnyPfannschmidt/borg,raxenak/borg