revlog: add a small cache of unfiltered chunk This can provides a massive boost to the reading of multiple revision and the computation of a valid delta chain. This greatly help operation like `hg log --patch`, delta computation (helping pull/unbundle), linkrev adjustment (helping copy tracing). A first round of benchmark for `hg log --patch --limit 1000` shows improvement in the 10-20% range on "small" repository like pypy or mercurial and large improvements (about 33%) for more complex ones like netbeans and mozilla's. These speeds up are consistent with the improvement to `hg pull` (from a server sending poor deltas) I saw benchmarking this last year. Further benchmark will be run during the freeze. I added some configuration in the experimental space to be able to further test the effect of various tuning for now. This feature should fit well in the "usage/resource profile" configuration that we should land next cycle. When it does not provides a benefit the overhead of the cache seem to be around 2%, a small price for the big improvement. In addition I believe we could shave most of this overhead with a more efficent lru implementation.