fix SetReadDeadline() has no effect after AcceptKCP() has been called

[shawwwn]

kcp-go/sess.go

Lines 913 to 918 in 88fc14a

	// AcceptKCP accepts a KCP connection
	func (l *Listener) AcceptKCP() (*UDPSession, error) {
	var timeout <-chan time.Time
	if tdeadline, ok := l.rd.Load().(time.Time); ok && !tdeadline.IsZero() {
	timeout = time.After(time.Until(tdeadline))
	}

Here var timeout is fixed, thus subsequent calls to SetDeadline() after AcceptKCP() will not be able to change the listener's timeout.

lis.SetDeadline(time.Now().Add(10 * time.Second))		// OK
conn, _ := lis.AcceptKCP()

go func() {
	<-time.After(5 * time.Second)
	lis.SetDeadline(time.Now().Add(10 * time.Second))	// Has no effect
}()

From a naive understanding of the go document, calling SetReadDeadline() should be able to extend the deadline of the underlying connection.
https://golang.org/pkg/net/#PacketConn