clfilter: make the revlog class responsible of all its iteration
This prepares changelog level filtering. We need the algorithms used in revlog to
work on a subset of revisions. To achieve this, the use of explicit range of
revision is banned. `range` and `xrange` calls are replaced by a `revlog.irevs`
method. Filtered super class can then overwrite the `irevs` method to filter out
revision.