stablerange: introduce a smarte version of stablerange for 'mergepoint' This implementation will using caching and smarter algorithm based on these cache. For now it inherit most of it code from the dummy version for convenience. The inheritance to the dummy version should be dropped eventually.

stablerange: introduce an intermediary abstract class for caching The 'branchpoint' and 'mergepoint' based stablerange class will need similar cache, we introduce an abstract class to take care of that first.