v10 will run 1 year after the upgrade to v9, to give time for any v8
processes to die. Until that point, the v10 upgrade will be tried by
every process but deferred, so added support for deferring upgrades.

The upgrade prevention lock file that will be used by v10 is not yet
implemented, so it does not yet defer.

Sponsored-by: Dartmouth College's Datalad project