KeepAlive enhancement.

If the server doesn't send keepalive for 20 seconds, generate an error.
fix #251, close #255

bot/basic/keepalive.go

@@ -1,15 +1,27 @@
 package basic
 
 import (
+	"time"
+
 	"github.com/Tnze/go-mc/data/packetid"
 	pk "github.com/Tnze/go-mc/net/packet"
 )
 
+const keepAliveDuration = time.Second * 20
+
+func (p *Player) resetKeepAliveDeadline() {
+	newDeadline := time.Now().Add(keepAliveDuration)
+	p.c.Conn.Socket.SetDeadline(newDeadline)
+}
+
 func (p *Player) handleKeepAlivePacket(packet pk.Packet) error {
 	var KeepAliveID pk.Long
 	if err := packet.Scan(&KeepAliveID); err != nil {
 		return Error{err}
 	}
+
+	p.resetKeepAliveDeadline()
+
 	// Response
 	err := p.c.Conn.WritePacket(pk.Packet{
 		ID:   int32(packetid.ServerboundKeepAlive),