• #522 — add receivePresence middleware hook
• #524 — deprecate sending sendPresence middleware errors to subscribers. The new behaviour will output errors to a new Backend error handler, which logs to console by default, and whose behaviour can be overridden by setting:
  new Backend({errorHandler: (error, context) => {}});
  To opt in to the new behaviour, set:
  new Backend({doNotForwardSendPresenceErrorsToClient: true});
  At the moment, this error handler will only handle these sendPresence middleware errors, but in the future may be expanded to handle other non-fatal ShareDB errors which are currently just logged.