ReplicatedPG: block reads on an object until the write is committed

Signed-off-by: Samuel Just <sam.just@inktank.com>
RadosModel: send racing read on write

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: remove the other backfill related flushes

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #728 from ceph/wip-6059

Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>