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.

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.

perf: adds cachekey utility to validate changelog+obsstore content We adds more helper about cache key to prepare the arrival of a cache that can be updated iteratively (similar to branchmap cache)

perf: adds some cache key helper on the obsstore class This will be useful to allow validating cache depending on obsstore without parsing the whole obsstore.

compat: only install the better filtered message for mercurial 4.2 and above The helper function does not exist in earlier version.