rgw/topic: add rgwrados::topic interface for topic metadata

rgw/topic: add rgwrados::topic interface for topic metadata

add a new interface for topic metadata that doesn't depend on metadata
backends. this low-level interface is used by both RadosStore and the
topic metadata handler

remove Driver::delete_bucket_topic_mapping() from sal because the omap
object is deleted internally by rgwrados::topic::remove()

remove the RGWRados::topics_pool_ctx member

Signed-off-by: Casey Bodley <cbodley@redhat.com>