nodemap: keep track of the docket for loaded data
To perform incremental update of the on disk data, we need to keep tracks of
some aspect of that data.
Differential Revision: https://phab.mercurial-scm.org/D7885
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}