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/

origin/tunnel.go

@@ -174,11 +174,11 @@ func ServeTunnelLoop(
 
 		config.Observer.SendReconnect(connIndex)
 
-		duration, ok := protocolFallback.GetBackoffDuration(ctx)
+		duration, ok := protocolFallback.GetMaxBackoffDuration(ctx)
 		if !ok {
 			return err
 		}
-		connLog.Info().Msgf("Retrying connection in %s seconds", duration)
+		connLog.Info().Msgf("Retrying connection in up to %s seconds", duration)
 
 		select {
 		case <-ctx.Done():