RGW Zipper - API Cleanups

RGW Zipper - API Cleanups

During the documentation pass for the Zipper API, a number of cleanups
were found: APIs that should be slightly different, or that were unused
entirely.  This is a rollup commit of all those cleanups.

- move get_multipart_upload() to Bucket
- remove unused defer_gc
- move create_bucket() into User
- rename get_bucket_info() to load_bucket() to match load_user()
- Remove read_bucket_stats()
  The codepaths using read_bucket_stats() used CLS data types, and the
  function is confusingly named.  Load the ent in load_bucket(), and use
  an alternative data structure to get size stats for the bucket.
- rename get_bucket_stats to read_stats
- Remove remove_metadata() from API
- remove copy_obj_data() from API
- rename get_obj_layout to dump_obj_layout
- use SAL range_to_ofs

Signed-off-by: Daniel Gryniewicz <dang@redhat.com>