V2 Netbird integration - UDP MIMO/SIMO, ICMPv6, DHCPv6 implementations (#106)

* begin adding udp.MuxHandler

* add udp MuxHandlerSIMO/MIMO

* add tcp rx shutdown

* icmpv6 client

* icmpv6 Client shared NDP/Echo preparation

* icmpv6 client ndp/echo split

* icmpv6 client ndp/echo split done

* icmpv6 adjustments

* add dhcpv6 stubs

* dhcpv4 preliminary revision

* add dns.NextLabel

* dns label name tweaks

* dns begin work on TCP client

* add dnstcp package

* apply gofmt changes

* add udp mux tests

* clean up, remove StackBig for now

* remove dnstcp so as to merged confident parts and we continue dnstcp work elsewhere

udp/conn.go

@@ -164,7 +164,7 @@ func (conn *Conn) Read(b []byte) (int, error) {
 			conn.mu.Unlock()
 			return 0, net.ErrClosed
 		}
-		n, err := conn.h.Read(b)
+		n, err := conn.h.ReadNext(b)
 		conn.mu.Unlock()
 		if n > 0 {
 			return n, err