lneto rework: Require explicit BackoffStrategy in all APIs (#116)

* make backoff explicit API

* fix tests to use explicit backoff

* fix examples with explicit tcp

x/xnet/stack-retrying.go

@@ -10,6 +10,9 @@ import (
 )
 
 func (s *StackAsync) StackRetrying(stackProtoBackoff lneto.BackoffStrategy) StackRetrying {
+	if stackProtoBackoff == nil {
+		panic("nil backoff to StackRetrying")
+	}
 	return StackRetrying{
 		block: s.StackBlocking(stackProtoBackoff),
 	}