switch to new sync/atomic helpers in go1.19 (#728)

These new helpers make the code a lot cleaner. I confirmed that the
simple helpers like `atomic.Int64` don't add any extra overhead as they
get inlined by the compiler. `atomic.Pointer` adds an extra method call
as it no longer gets inlined, but we aren't using these on the hot path
so it is probably okay.

handshake_ix.go

@@ -1,7 +1,6 @@
 package nebula
 
 import (
-	"sync/atomic"
 	"time"
 
 	"github.com/flynn/noise"
@@ -51,7 +50,7 @@ func ixHandshakeStage0(f *Interface, vpnIp iputil.VpnIp, hostinfo *HostInfo) {
 	}
 
 	h := header.Encode(make([]byte, header.Len), header.Version, header.Handshake, header.HandshakeIXPSK0, 0, 1)
-	atomic.AddUint64(&ci.atomicMessageCounter, 1)
+	ci.messageCounter.Add(1)
 
 	msg, _, _, err := ci.H.WriteMessage(h, hsBytes)
 	if err != nil {