many recompute improvements

many recompute improvements

I've lost track of them all, but it includes:

* Using the same key backend as was used in the original computation.
* Fixing bug that prevented updating the source file key in the compute
  state
* Handling --reproducible and --unreproducible.
* recompute --original of a file using VURL, when the result is
  different, but the key remains the same, makes the object file
  be updated with the new content
* Detecting some other ways the program behavior can change, just for
  completeness.
* Also adds --backend to addcomputed.