nodemap: write nodemap data on disk
Let us start writing data on disk (so that we can read it from there later).
This series of changeset is going to focus first on having data on disk and
updating it.
Right now the data is written right next to the revlog data, in the store. We
might move it to cache (with proper cache validation mechanism) later, but for
now revlog have a storevfs instance and it is simpler to us it. The right
location for this data is not the focus of this series.
Differential Revision: https://phab.mercurial-scm.org/D7835