changegroup: remove _clnodetorev cgpacker._clnodetorev is a glorified cache/index of changelog nodes to revision numbers. I'm not sure why it exists. Maybe performance? But its presence is making refactoring of this code more complicated than it needs to be. This commit removes the cache and replaces it with direct lookups against the changelog. If this cache was for performance reasons, we should be able to restore it easily enough... after the changegroup refactor is complete. Differential Revision: https://phab.mercurial-scm.org/D4189