Verify membership writes under concurrency

**Property.** However many commands write one person's memberships at once, and however PostgreSQL interleaves their statements (a request can be cancelled at any step), no write is rolled back as a deadlock victim, no write waits inside the exclusion check for another transaction's unfinished row (the writes of one person never run at the same time), no two overlapping memberships of one person in one team are both committed, "already exists" is only said when a committed membership overlaps, and a change commits only if it was legal from the membership it really replaced. The commands that change who is on the admin list take its lock before any person's, so the order of the locks never makes two commands wait for each other.