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>
ReplicatedPG: remove unused RWTracker::ObjState::clear

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG.h: while there cannot be a read in progress, there may be a read blocked

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #706 from ceph/wip-6059

Wip 6059

Partly-reviewed-by: Greg Farnum <greg@inktank.com>
Partly-reviewed-by: Sage Weil <sage@inktank.com>