Implement retry-with-delays event reaction and corresponding sbuild workflow changes

From https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/dep-wait.html, the Delay task has already been implemented. So what's left to be implemented in the "retry-with-delays" event reaction and the addition of the "retry_delays" field in the sbuild workflow.

This is a simple work-around to the possible unavailability of some build dependencies until they had the time to get built (cf #374 (closed) and #462 (closed) for the initial tickets).