PING a quiet IRC listener client before closing it (closes #155)

serve closed a connection after five minutes with nothing read and never
sent a PING, so a quiet registered client lost its session. It now sends
PING :<server> after pingInterval with nothing read, and closes the
connection when nothing is read within pongDeadline after that. Every
line read starts both waits again. readTimeout is removed.

Unregistered connections are sent the PING too, so a PONG before
registration is now ignored instead of answered with ERR_NOTREGISTERED.

Model: opus-5-5

README.md

@@ -2667,6 +2667,9 @@ IRC_LISTEN_ADDR: ""
   An optional `PASS` before registration sets the session password (minimum 8
   characters). Registering creates a session without a hashcash stamp; closing
   the connection ends the session, as `QUIT` does.
+- **PING**: a connection that sends nothing for 90 seconds is sent a `PING`, and
+  is closed if it sends nothing in the 30 seconds after that; any line, such as
+  the client's `PONG`, starts both waits again.
 - **CAP negotiation**: `CAP LS` gets an empty capability list and `CAP END` is
   ignored, for compatibility with modern clients.
 - **Channel prefixes**: Channels must start with `#`. `JOIN` prepends it if