addcomputed --fast and --unreproducible working

addcomputed --fast and --unreproducible working

For these, use VURL and URL keys, with an "annex-compute:" URI prefix.

These URL keys will look something like this:

	URL--annex-compute&cbar4,63pconvert,3-f4d3d72cf3f16ac9c3e9a8012bde4462

Generally it's too long so most of it gets md5summed. It's a little
ugly, but it's what fell out of the existing URL key generation
machinery. I did consider special casing to eg
"URL--annex-compute&c4d3d72cf3f16ac9c3e9a8012bde4462". But it seems at
least possibly useful that the name of the file that was computed is
visible and perhaps one or two words of the git-annex compute command
parameters.

Note that two different output files from the same computation will get
the same URL key. And these keys should remain stable.