devel: add a config field to force dates to timestamp 0 Add a new config field named default-date under the devel section to force all implicits date to a specific value. If a explicit date is passed, it will override the default. This patch only affect changesets. Other usages (blackbox, obsmarkers) are updated in later patchs. The test runner is setting a bunch of alias to force the '--date' argument. We will replace theses aliases in a later patch.