change "last signoff" tracking to "last seen"

Explicit quit and ping timeout behave the same way,
but reattach after abandoning/losing the previous session
(without the break being detected server-side) is more aggressive
about replaying missed messages, at the cost of potential duplication.

irc/commands.go

@@ -65,8 +65,9 @@ func (cmd *Command) Run(server *Server, client *Client, session *Session, msg ir
 		session.idletimer.Touch()
 	}
 
-	if !exiting && client.registered && !cmd.leaveClientIdle {
-		client.Active(session)
+	// TODO: eliminate idletimer entirely in favor of this measurement
+	if client.registered {
+		client.Touch(!cmd.leaveClientIdle, session)
 	}
 
 	return exiting