rgw: read / write bucket attributes from bucket instance

and not from bucket entry point.

Fixes: #5770

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: track bucket instance oid

We now keep the bucket instance oid in rgw_bucket. The reason
we need it is that the bucket might have been created before
the entrypoint / bucket instance separation.

Fixes: #5790

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: set bucket attrs are a bucket instance meta operation

Need to do the action through the bucket instance handler
and not through the bucket handler, otherwise it's wrongly
recorded (and wrongly replayed, ouch).

Fixes: #5791

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