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
Add some timers to find hotspots more easily
Change the global stats thing

Now we just print the list of read/write structured files
Do not evaluate the format string when debug is not set
Add stats about external command calls
Merge branch 'master' of github.com:OCamlPro/opam

Conflicts:
	src/client/opamState.ml
Merge branch 'master' of github.com:samoht/opam
Fix forward compatibility of opam-version in metadata files

If a package/compiler contains an opam-version stricly greater than the current OPAM version, skip the file.
If ~/.opam/config contains an opam-version stricly greater than the current OPAM version, the fail and tell the user to upgrade.
Update the dev-manual to specify when opam-config is necessary

answer: it is necessary in user-facing files, such as package and compiler metadata files.
Merge branch 'master' of github.com:OCamlPro/opam

Conflicts:
	src/core/opamGlobals.ml