common: ghobject get_filestore_key* use hobject counterpart

The get_filestore_key* methods are changed to just call the
corresponding hobject methods instead of providing an identical
implementation.

Signed-off-by: Loic Dachary <loic@dachary.org>
common: document sort order rationale

Intuition differs regarding the sort order of the ghobject shard and
generation. Document the rationale for the chosen sort order.

Signed-off-by: Loic Dachary <loic@dachary.org>