revlog: deprecate the _nodecache attribute (API)
This attribute have been the same object for a long time. Code should directly
access the nodemap in case of need. (Or the final API when this will be done).
This work is part of a refactoring to unify the revlog index and the nodemap.
This unification prepare the use of a persistent nodemap.
Differential Revision: https://phab.mercurial-scm.org/D7318