obsolete: introduce caches for all meaningful sets This changeset introduces caches on the `obsstore` that keeps track of sets of revisions meaningful for obsolescence related logics. For now they are: - obsolete: changesets used as precursors (and not public), - extinct: obsolete changesets with osbolete descendants only, - unstable: non obsolete changesets with obsolete ancestors. The cache is accessed using the `getobscache(repo, '<set-name>')` function which builds the cache on demand. The `clearobscaches(repo)` function takes care of clearing the caches if any. Caches are cleared when one of these events happens: - a new marker is added, - a new changeset is added, - some changesets are made public, - some public changesets are demoted to draft or secret. Declaration of more sets is made easy because we will have to handle at least two other "troubles" (latecomer and conflicting). Caches are now used by revset and changectx. It is usually not much more expensive to compute the whole set than to check the property of a few elements. The performance boost is welcome in case we apply obsolescence logic on a lot of revisions. This makes the feature usable!