node: replace nullid and friends with nodeconstants class [WIP] The introduction of 256bit hashes require changes to nullid and other constant magic values. Start pushing them down from repository and revlog where sensible. Note: this review contains two parts for correctness checks that need to be removed before the final merge. There are a couple of asserts in the manifest classes etc to ensure that the nodeconstants argument is sane. This ensured the correct API update. The other part is commenting out the legacy names in node.py. That will move under an environment flag for now and is not intended for the repo as is. The rest of the changeset is reviewable. Differential Revision: https://phab.mercurial-scm.org/D9465