rgw: don't modify rgw_bucket for data extra pool

rgw: don't modify rgw_bucket for data extra pool

This cleans up the interface, as we don't have this ugly implicit
structure modification when we deal with object that resides on the data
extra pool. Replcae lot's of calls to get_obj_bucket_and_oid_key() to a
cleaner get_obj_ref() that uses a struct to hold the updated fields, and
update the relevant callers.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>