nodemap: use an intermediate "docket" file to carry small metadata
This intermediate file will make mmapping, transaction and content validation
easier. (Most of this usefulness will arrive gradually in later changeset). In
particular it will become very useful to append new data are the end of raw file
instead of rewriting on the file on each transaction.
See in code comments for details.
Differential Revision: https://phab.mercurial-scm.org/D7838