chunks index archive: remove all tar and compression related stuff and just use separate files in a directory
chunks index archive: remove all tar and compression related stuff and just use separate files in a directory

the compression was quite cpu intensive and didn't work that great anyway.
now the disk space usage is a bit higher, but it is much faster and less hard on the cpu.

disk space needs grow linearly with the amount and size of the archives, this
is a problem esp. if one has many and/or big archives (but this problem existed
before also because compression was not as effective as I believed).

the tar archive always needed a complete rebuild (and thus: decompression
and recompression) because deleting outdated archive indexes was not
possible in the tar file.

now we just have a directory chunks.archive.d and keep archive index files
there for all archives we already know.
if an archive does not exist any more in the repo, we just delete its index file.
if an archive is unknown still, we fetch the infos and build a new index file.

when merging, we avoid growing the hash table from zero, but just start
with the first archive's index as basis for merging.

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