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/httpserver/main.go

@@ -232,6 +232,7 @@ func run() (err error) {
 			RxBuf:             make([]byte, mtu),
 			TxBuf:             make([]byte, mtu),
 			TxPacketQueueSize: 3,
+			RWBackoff:         tcpBackoff,
 		})
 		err = stack.ListenTCP4(&conn, svPort)
 		if err != nil {
@@ -363,3 +364,15 @@ 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)
+}