Simplify and speedup repo update

All the md5s were recomputed at each 'opam update', which is unnecessary since we already
have a URL file. Rebuilding the package cache is still expensive (until the OS cache is full) though

Bench with
```
sudo sysctl vm.drop_caches=3
time opam update
```
Cleanup: no need for preload anymore

we don't have concurrency issues anymore, so it's fine to get the repo
state cache lazily (as long as we don't yield in the middle)
Merge pull request #1893 from OCamlPro/faster-update

Simplify and speedup repo update, don't recompute md5s each time