Cache the repository priority and the corresponding prefixes regarding packages and compilers

compiler index is in ~/.opam/repo/compiler-index
package index is in ~/.opam/repo/package-index

~/.opam/repo/index is now deprecated, and editing this file to modify package priority manually is not supported anymore. Nobody was using it and it was quite a pain to maintain for no good particular reasons.
Experimental extra-fast init/update

We keep in the global state only the metadata of installed packages. So ne need to have a costly synchronization on each update.

Very untested, but at least tested on 'opam init --no-setup && opam list` wich becomes increadibly fast. Need to think a little more about the corner cases now ...
Read the cached indexes instead of computing them on every opam invocation