[WIP] casync: reuse seed cache for multiple checkouts

[giuseppe]

it allows to reuse a seed cache among different checkouts.

It enables this use case:

# The cache is empty, we retrieve all the chunks from the store
$ casync extract --seed-cache cache  --cache cache \
   --store=http://localhost:8000/default.castr  --seed-output=no i.caidx checkout_1

# We have created a checkout, fill the seed-cache
$ casync extract --seed-cache $(pwd)/cache  --cache $(pwd)/cache /default.castr \
 --dry-run --seed-output=yes  i.caidx t

# Now the seed cache is filled, we can reuse it without fetching any data from
# the remote store
$ casync extract --seed-cache cache  --cache cache \
  --seed-output=no i.caidx checkout_2

Closes: https://github.com/systemd/casync/issues/209

[giuseppe]

does not really solve the issue, closing for now