Implemented Commandline Download Speedtest (#2064)

Added the net/speedtest package that contains code for starting up a
speedtest server and a client. The speedtest command for starting a
client takes in a duration for the speedtest as well as the host and
port of the speedtest server to connect to. The speedtest command for
starting a server takes in a host:port pair to listen on.

Signed-off-by: Aaditya Chaudhary <32117362+AadityaChaudhary@users.noreply.github.com>

net/speedtest/speedtest.go

@@ -0,0 +1,88 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package speedtest contains both server and client code for
+// running speedtests between tailscale nodes.
+package speedtest
+
+import (
+	"time"
+)
+
+const (
+	blockSize       = 32000                 // size of the block of data to send
+	MinDuration     = 5 * time.Second       // minimum duration for a test
+	DefaultDuration = MinDuration           // default duration for a test
+	MaxDuration     = 30 * time.Second      // maximum duration for a test
+	version         = 1                     // value used when comparing client and server versions
+	increment       = time.Second           // increment to display results for, in seconds
+	minInterval     = 10 * time.Millisecond // minimum interval length for a result to be included
+	DefaultPort     = 20333
+)
+
+// config is the initial message sent to the server, that contains information on how to
+// conduct the test.
+type config struct {
+	Version      int           `json:"version"`
+	TestDuration time.Duration `json:"time"`
+	Direction    Direction     `json:"direction"`
+}
+
+// configResponse is the response to the testConfig message. If the server has an
+// error with the config, the Error variable will hold that error value.
+type configResponse struct {
+	Error string `json:"error,omitempty"`
+}
+
+// This represents the Result of a speedtest within a specific interval
+type Result struct {
+	Bytes         int           // number of bytes sent/received during the interval
+	IntervalStart time.Duration // duration between the start of the interval and the start of the test
+	IntervalEnd   time.Duration // duration between the end of the interval and the start of the test
+	Total         bool          // if true, this result struct represents the entire test, rather than a segment of the test
+}
+
+func (r Result) MBitsPerSecond() float64 {
+	return r.MegaBits() / (r.IntervalEnd - r.IntervalStart).Seconds()
+}
+
+func (r Result) MegaBytes() float64 {
+	return float64(r.Bytes) / 1000000.0
+}
+
+func (r Result) MegaBits() float64 {
+	return r.MegaBytes() * 8.0
+}
+
+func (r Result) Interval() time.Duration {
+	return r.IntervalEnd - r.IntervalStart
+}
+
+type Direction int
+
+const (
+	Download Direction = iota
+	Upload
+)
+
+func (d Direction) String() string {
+	switch d {
+	case Upload:
+		return "upload"
+	case Download:
+		return "download"
+	default:
+		return ""
+	}
+}
+
+func (d *Direction) Reverse() {
+	switch *d {
+	case Upload:
+		*d = Download
+	case Download:
+		*d = Upload
+	default:
+	}
+}