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.

firewall.go

@@ -879,7 +879,7 @@ func parsePort(s string) (startPort, endPort int32, err error) {
 	return
 }
 
-//TODO: write tests for these
+// TODO: write tests for these
 func setTCPRTTTracking(c *conn, p []byte) {
 	if c.Seq != 0 {
 		return