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.

Reviewed-by: David Zafman <david.zafman@inktank.com>
Signed-off-by: Loic Dachary <loic@dachary.org>
common: document ghobject sort order rationale

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

Reviewed-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #652 from dachary/wip-ghobjects

common: ghobject sort order & get_filestore_key