librbd: add options to enable balanced or localized reads for snapshots

librbd: add options to enable balanced or localized reads for snapshots

Since snapshots never change, it's safe to read from replicas for them.
A common use for this would be reading from a parent snapshot shared by
many clones.

Convert LibrbdWriteback and AioRead to use the ObjectOperation api
so we can set flags. Fortunately the external wrapper holds no data,
so its lifecycle doesn't need to be managed.

Include a simple workunit that sets the flags in various combinations
and looks for their presence in the logs from 'rbd export'.

Fixes: #3064
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>