Add design for coordinating sub-workflows

This nails down a number of details that were previously underspecified. After extensive discussion, the only new features that ended up being needed were the debusine:promise bare data collection item, a prefix field in the task data for some workflows, and some additional work request helper methods. The protocol for calling workflow populate methods also changes slightly. Everything else is based on the existing system of event reactions and lookups.

Part of #397 (closed).