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.