TUN-3902: Add jitter to backoffhandler

Jitter is important to avoid every cloudflared in the world trying to
reconnect at t=1, 2, 4, etc. That could overwhelm the backend. But
if each cloudflared randomly waits for up to 2, then up to 4, then up
to 8 etc, then the retries get spread out evenly across time.

On average, wait times should be the same (e.g. instead of waiting for
exactly 1 second, cloudflared will wait betweeen 0 and 2 seconds).

This is the "Full Jitter" algorithm from https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/

cmd/cloudflared/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"math/rand"
 	"strings"
 	"time"
 
@@ -43,6 +44,7 @@ var (
 )
 
 func main() {
+	rand.Seed(time.Now().UnixNano())
 	metrics.RegisterBuildInfo(BuildTime, Version)
 	raven.SetRelease(Version)