os: use transparent comparator in ObjectStore::getattrs()

os: use transparent comparator in ObjectStore::getattrs()

for two reasons:

- better performance when looking on in the return map if the key
  is not a string, as we don't need to create a temporary string
  as the key
- improve the performance of crimson::AlienStore, as the latter
  uses the transparent comparator. as, without this change, we'd
  have to perform a deep copy to fill up the returned map with
  its non-transparent-comparator version.

Signed-off-by: Kefu Chai <kchai@redhat.com>