ghobject_t: change sort order (max, shard, hobj, gen)

ghobject_t: change sort order (max, shard, hobj, gen)

Go from (hobj, shard, gen) -> (max, shard, hobj, gen)

This makes the ghobject_t's sort in a way that groups them by PG, which
will likely be useful in the future for ObjectStore implementations.
Notably, we get a ghobject_t MAX value that is distinct from the
hobject_t one.

Signed-off-by: Sage Weil <sage@redhat.com>