perf: use the cache to compute the obsolete set. The official "obsolete" computation is switch to using the cache. This provide noticable speed for operation that does not need to actually access the obs markers. The part relating to obsolete changeset computation disappear from the execution profile when it is used.

perf: warm the cache after all transactions This is the simplest way to ensure the data are up to date.

perf: adds a cache to know if obsmarkers might affect a revision Phase information still needs to be thrown in the mix to compute the final information, but skipping reading the obsstore for most operation is a large win. Usage of this cache arrives in the next changeset.