With four fresh sessions in one repository the coordinator must be able to tell
which peers are its workers without assuming: list the sessions in this cwd
excluding itself, probe each, recruit the responders up to three, never charter a
session that has not answered, and record the recruited ids in the ledger so a
later cycle or a replacement coordinator can tell who is who. This also covers the
case where an extra session (a planning session left open) is live in the repo.
Christopher Williams
48dca3e271