obscache: skip the cache entirely if not up to date The current update code has some race condition windows around updating. But we also ensure the cache are up to date with every transaction. So outdated cache mean another client have been mudding the repository but things will get back in place at the next transaction. So we just skip using the cache when not up to date. This is not the best we could do. But this is good enough for now.

readme: mention that some improvement are enabled for 4.2 only

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.