conn: fix goroutine leak

As is, soju leaks goroutines on client disconnects, because the closure
started as goroutine in newConn never finishes. It gets stuck in the
for loop annotated as "draining the outgoing channel", because the
outgoing channel is in fact never closed.

This commit fixes the issue by calling conn.Close rather than
conn.conn.Close, which closes not only the underlying net.Conn (in
c.conn), but also the channel.

conn.go

@@ -157,7 +157,7 @@ func newConn(srv *Server, ic ircConn, options *connOptions) *conn {
 				break
 			}
 		}
-		if err := c.conn.Close(); err != nil && !errors.Is(err, net.ErrClosed) {
+		if err := c.Close(); err != nil && !errors.Is(err, net.ErrClosed) {
 			c.logger.Printf("failed to close connection: %v", err)
 		} else {
 			c.logger.Debugf("connection closed")