New method telling if changesets are obsolete

Mergeability checks such as the ones failing in heptapod#670 will need to detect early that they are running on an obsolete changeset. Also it is a long-standing user request to display obsolescence in the UI (heptapod#23, heptapod#646).

As with all Mercurial-specific gRPC methods, we will need a new version of the Ruby client lib, and in this case probably a new service (MercurialChangesetService)