DNM: simplify ObjectStore interface

- first, push the upgrade stuff into ObjectStore and into FileStore specifically
  OSD has no business doing this stuff.
- Then kill it.  I don't trust the colleciton_add/remove stuff anyway.
- Remove collection_{add,remove} from the interface
- use a attr to mark collection deletion instead of renaming it
- remove collection_rename

This simplifies the interface surface.  Mainly, it gets rid of the add/remove
link/unlink stuff so that there are just two operations we nee dto support:

 collection_move
 collection_move_rename

...which makes life less complicated!
which line?

we want to mirror what previously woudl happen when we looped between the ADD and REMOVE