use a net.PacketConn instead of a net.UDPConn in Server and Session

conn.go

@@ -0,0 +1,39 @@
+package quic
+
+import (
+	"net"
+	"sync"
+)
+
+type connection interface {
+	write([]byte) error
+	setCurrentRemoteAddr(net.Addr)
+	RemoteAddr() net.Addr
+}
+
+type conn struct {
+	mutex sync.RWMutex
+
+	pconn       net.PacketConn
+	currentAddr net.Addr
+}
+
+var _ connection = &conn{}
+
+func (c *conn) write(p []byte) error {
+	_, err := c.pconn.WriteTo(p, c.currentAddr)
+	return err
+}
+
+func (c *conn) setCurrentRemoteAddr(addr net.Addr) {
+	c.mutex.Lock()
+	c.currentAddr = addr
+	c.mutex.Unlock()
+}
+
+func (c *conn) RemoteAddr() net.Addr {
+	c.mutex.RLock()
+	addr := c.currentAddr
+	c.mutex.RUnlock()
+	return addr
+}