receiveEvent(token: string): Promise<void>
Applies the signed event token from a user's home, for
POST /api/federation/events: update brings their account in step,
revoke ends their sessions here. Only an instance some account here comes
from, and not blocked, may send one, about one of those accounts, and at
most EVENT_LIMIT's rate. The sender gives up on an event at a 4xx
other than 429, and tries it again later otherwise.