v7.1.0

• New: watch() registers a presence watch and receives pushed updates.
  On the mailbox socket, watch() now sends one presence-watch request for
  the first 16 watched accounts. It sends the request when the socket
  connects, at each change of that set, and again every 120 s
  (PRESENCE_WATCH_RENEWAL_MILLISECONDS). The answer is the current presence
  of those accounts. After that, the Signal Protocol Relay pushes a
  presence-update frame for each change, and watch() calls the listener
  as it does for a changed read. The 30 s poll now reads only the accounts
  past the first 16. After a watch fails, or has no answer in 10 s, the poll
  reads every account every 30 s, and the watch tries again 120 s later. A
  new presence key from grant() for a watched account sends the watch
  again. A malformed presence-update frame reconnects the socket. The wake
  client and the HTTP carrier have no watch. A Relay without
  presence-watch closes the socket with 1008. The client then reads after
  30 s and tries the watch again after 120 s.