os/bluestore: reimplement zone backrefs

os/bluestore: reimplement zone backrefs

Implement a (zone, offset) -> oid reference.  The offset is the first
offset referenced by this object in this zone; subsequent extents in the
same zone will not generate additional refs.  The ref will be removed only
when *all* refs for that object to that zone are gone.

- minimal metadata per zone (one ref to each object involved
- (zone, offset) keys are ordered for efficient sequential cleaning

Signed-off-by: Sage Weil <sage@newdream.net>