Handle synchronization points and workflow callbacks in scheduler

Part of #276 (closed).

https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/workflows.html defines "synchronization points" and "workflow callbacks" as special kinds of work requests, and calls for special handling of them in the scheduler.

• implement synchronization points (can be done immediately)
• implement scheduler handling of synchronization points (!774 (merged))
• implement workflow callbacks (depends on #339 (closed)) (!737 (merged))
• implement scheduler handling of workflow callbacks (!778 (merged))