Ceph » rgw » cleanup

from https://github.com/ceph/ceph/pull/56583#issuecomment-2037285274:

i have a general distaste for Bucket::merge_and_store_attrs() because it can be implemented purely in terms of other public Bucket functions. that tells me it doesn't need to be part of our virtual backend interface, and could just be a free function instead. when it's part of the virtual interface, we have to duplicate this for each backend - and i see that dbstore's DBBucket::merge_and_store_attrs() has the same bug as RadosBucket