cache: introduce a changelogsourcebase class This abstract class will help code that need a cache tracking the changelog content (eg: the branchmap cache). The cache key used is the same as what the branchmap uses. This code was headed for core during the 4.3 cycle but never made it there. So we starts with a copy in the evolve repository.

cache: introduce an abstract class for cache we can upgrade incrementally Right now each class implements their own mechanism for validation, and update. We start introducing abstract class to ultimately allow more unification of the cache code. The end goal of this series is to introduce a cache for some obsolescence property, not to actually implement the cache. However, taking advantage of adding a new cache to introduce the abstract class seems like a win. This code was headed for core during the 4.3 cycle but never made it there. So we starts with a copy in the evolve repository.

depth: basic debugdepth implementation and test This will come very handy to validate more advanced implementation later.

topics: add a config to reject draft changeset without topic on a server This patch adds a new config option experimental.topic-mode.server which if sets to True, the server won't accept any draft changeset without topic on it. In case both `experimental.topic-mode.server` and `experimental.topic.publish-bare-branch` are set to True, the enforce-topic one is respected. Tests are added for it. The CHANGELOG file is also updated mentioning about the config option.