nodemap: track the maximum revision tracked in the nodemap
We need a simple way to detect when the on disk data contains less revision
than the index we read from disk. The docket file is meant for this, we just had
to start tracking that data.
We should also try to detect strip operation, but we will deal with this in
later changesets. Right now we are focusing on defining the API for index
supporting persistent nodemap.
Differential Revision: https://phab.mercurial-scm.org/D7888