Introduce opam-root-version

[dra27]

Discussed on 1 Apr, introduce opam-root-version for the root and switch config files which is checked for taking out write locks. opam-version is checked for taking out read locks. So after root upgrade, $OPAMROOT/config would contain:

opam-version: "2.0"
opam-root-version: "2.1"

opam-state 2.0.x should be updated to internally recognise opam-root-version (i.e. no change in API in OpamFile) and ignore fields it doesn't recognise. The changes we've made for 2.1 mean that it should "just work" to have readable roots to 2.0.

As always, a given version of opam would never attempt to read a file with opam-version greater than itself.