docket: move the uid logic in the `revlogutils.docket` module
We want to use it for revlog-v2 (& Co), it seems more logical to have the logic
lives inside the `docket` file than the `nodemap` file.
Differential Revision: https://phab.mercurial-scm.org/D10755
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors