clfilter: make localpeer use a repo with "unserved" filter This changeset installs a broad filter on most repos used for serving. This removes the need to use the `visiblehead`/`visiblebranchmap` functions, and ensures that changesets we should not serve are in fact never served. We do not use filtering on hgweb yet, as there is still a number of issues to solve there.

performance: speedup computation of extinct revisions In their current state, revset calls can be very costly, as we test predicates on the entire repository. This change drops revset calls in favor of direct testing of the phase of changesets. Performance test on my Mercurial checkout - 19857 total changesets, - 1584 obsolete changesets, - 13310 obsolescence markers. Before: ! extinct ! wall 0.015124 After: ! extinct ! wall 0.009424 Performance test on a Mozilla central checkout: - 117293 total changesets, - 1 obsolete changeset, - 1 obsolescence marker. Before: ! extinct ! wall 0.032844 After: ! extinct ! wall 0.000066