clfilter: introduce a "unserver" filtering mode This mode is for repository used as a server. It filter secret and hidden changeset out. It is put to use in later changeset.

clfilter: add a cache on repo for set of revision to filter for a given set. Recomputing the filtered revisions at every access to changelog is far too expensive. This changeset introduce a cache for this information. This cache is hold by the repository (unfiltered repository) and invalidated when necessary. This cache is not a protected attribute (leading _) because some logic that invalidate it is not held by the local repo itself.

clfilter: add actual repo filtering mechanism We add a `filtered` method on repo. This method return an instance of `repoview` that behaves exactly as the original repository but with a filtered changelog attribute. Filters are identified by a "name". Planned filter are `unserved`, `hidden` and `mutable`. Filtering the repository in place what out of question as it wont not allows multiple thread to share the same repo. It would makes control of the filtering scope harder too. See the `repoview` docstring for details. A mechanism to compute filtered revision is also installed. Some caches will be installed in later commit.