rgw: unlink bucket from user on metadata rm bucket:< bucket>

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: some more internal api cleanups

Use of rgw_bucket when referring to the bucket instance,
use bucket name when referring to the bucket entry point.
Also, remove bucket input param where not needed (internally
was using the bucket structure from the bucket info).

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

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: tie metadata put to bucket link/unlink

and lot's of constifying

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: unlink/link don't always update entry point

Some operations already update the entry point, so no
need to do it again.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: log in the same shard for bucket entry point and instance

We'd like to have bucket entry point and instance info at the same
log shard, so that we can process them in order.

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