Make an IRC listener connection follow its session's nick (closes #184)

When the session's nick was changed from another client, such as the
web client over the HTTP API, the IRC listener connection kept the old
nick and sent its messages, joins and later NICK changes under it. Now,
whenever the relay passes on a NICK line, and after the connection's
own NICK, the connection takes the nick its session holds in the
database, through loadSessionNick. It does not go by the line's old
nick, as the connection may have taken that nick after another user
gave it up. As the relay goroutine can now change the nick, the command
handlers read it under the connection's lock, through currentNick.

Model: opus-5-5

internal/ircserver/relay.go

@@ -92,7 +92,7 @@ func (c *Conn) drainQueue(ctx context.Context) bool {
 //
 //nolint:cyclop // dispatch table
 func (c *Conn) deliverIRCMessage(
-	_ context.Context,
+	ctx context.Context,
 	msg *db.IRCMessage,
 ) {
 	command := msg.Command
@@ -120,7 +120,7 @@ func (c *Conn) deliverIRCMessage(
 	case command == irc.CmdPart:
 		c.deliverPart(msg, text)
 	case command == irc.CmdNick:
-		c.deliverNickChange(msg, text)
+		c.deliverNickChange(ctx, msg, text)
 	case command == irc.CmdQuit:
 		c.deliverQuitMsg(msg, text)
 	case command == irc.CmdTopic:
@@ -230,11 +230,18 @@ func (c *Conn) deliverPart(msg *db.IRCMessage, text string) {
 	}
 }
 
-// deliverNickChange sends a NICK change notification.
+// deliverNickChange sends a NICK change notification. The
+// change may be its own session's, made from another client,
+// so the connection first takes the nick its session holds
+// now. It does not go by the line's old nick: the connection
+// may have taken that nick after another user gave it up.
 func (c *Conn) deliverNickChange(
+	ctx context.Context,
 	msg *db.IRCMessage,
 	newNick string,
 ) {
+	c.loadSessionNick(ctx)
+
 	prefix := msg.From + "!" + msg.From + "@*"
 
 	c.send(FormatMessage(prefix, "NICK", newNick))