rgw: Remove `RGWSI_RADOS` from `RGWSI_MDLog`

rgw: Remove `RGWSI_RADOS` from `RGWSI_MDLog`

Simply use the RADOS handle and `rgw_rados_ref` directly.

Also move `async_processor` out from `RGWSI_RADOS` and into
`RGWServices_Def`. This is as good a place as it for any, for now, as
it's reachable by everyone who needs it and exists through the
lifetime of the process.

Eventually it's going to go away due to coroutinization, anyway.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>