rgw: batch and process bucket instances

rgw: batch and process bucket instances

Sort through and batch bucket instances so that multiple calls to reading
current bucket info and locking can be avoided. For the most trivial case when
the bucket is already deleted we exit early with all the stale instances. When
the bucket reshard is in progress we only process the stale entries with status
done, if the bucket is available for locking then we lock down and mark the
other instances as well.

Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
(cherry picked from commit fb9c0497621fce34283094cfe260480f9122573f)

 Conflicts:
	src/rgw/rgw_bucket.cc
Get rid of the following c++17isms:
- split_tenant auto return type -> trailing return type
- tuple destructuring bind for split tenant with std::tie