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

* make backoff explicit API

* fix tests to use explicit backoff

* fix examples with explicit tcp

examples/min-working-example/main-mwe.go

@@ -102,6 +102,7 @@ func run(ctx context.Context, stack *xnet.StackAsync) error {
 			NanoTime:           nanotime,
 			EstablishedTimeout: tcpEstablishedTimeout,
 			ClosingTimeout:     tcpCloseTimeout,
+			NewBackoff:         func() lneto.BackoffStrategy { return tcpBackoff },
 		},
 	})
 
@@ -178,3 +179,15 @@ func stackBackoff(consecutiveBackoffs uint) time.Duration {
 	}
 	return 10 * time.Millisecond
 }
+
+func tcpBackoff(consecutiveBackoffs uint) time.Duration {
+	const (
+		minWait        = uint32(time.Microsecond)
+		maxWait        = 5 * uint32(time.Millisecond)
+		maxShift       = 22
+		_overflowCheck = minWait << maxShift
+	)
+	shifted := minWait << min(consecutiveBackoffs, maxShift)
+	wait := min(shifted, maxWait)
+	return time.Duration(wait)
+}