TUN-1099: Bring back changes in 2018.10.1

vendor/github.com/jonboulle/clockwork/README.md

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+clockwork
+=========
+
+[![Build Status](https://travis-ci.org/jonboulle/clockwork.png?branch=master)](https://travis-ci.org/jonboulle/clockwork)
+[![godoc](https://godoc.org/github.com/jonboulle/clockwork?status.svg)](http://godoc.org/github.com/jonboulle/clockwork) 
+
+a simple fake clock for golang
+
+# Usage
+
+Replace uses of the `time` package with the `clockwork.Clock` interface instead.
+
+For example, instead of using `time.Sleep` directly:
+
+```
+func my_func() {
+	time.Sleep(3 * time.Second)
+	do_something()
+}
+```
+
+inject a clock and use its `Sleep` method instead:
+
+```
+func my_func(clock clockwork.Clock) {
+	clock.Sleep(3 * time.Second)
+	do_something()
+}
+```
+
+Now you can easily test `my_func` with a `FakeClock`:
+
+```
+func TestMyFunc(t *testing.T) {
+	c := clockwork.NewFakeClock()
+
+	// Start our sleepy function
+	my_func(c)
+
+	// Ensure we wait until my_func is sleeping
+	c.BlockUntil(1)
+
+	assert_state()
+
+	// Advance the FakeClock forward in time
+	c.Advance(3)
+
+	assert_state()
+}
+```
+
+and in production builds, simply inject the real clock instead:
+```
+my_func(clockwork.NewRealClock())
+```
+
+See [example_test.go](example_test.go) for a full example.
+
+# Credits
+
+clockwork is inspired by @wickman's [threaded fake clock](https://gist.github.com/wickman/3840816), and the [Golang playground](http://blog.golang.org/playground#Faking time)