Update vendor (#1384)

vendor/github.com/slack-go/slack/internal/backoff/backoff.go

@@ -0,0 +1,62 @@
+package backoff
+
+import (
+	"math/rand"
+	"time"
+)
+
+// This one was ripped from https://github.com/jpillora/backoff/blob/master/backoff.go
+
+// Backoff is a time.Duration counter. It starts at Min.  After every
+// call to Duration() it is multiplied by Factor.  It is capped at
+// Max. It returns to Min on every call to Reset().  Used in
+// conjunction with the time package.
+type Backoff struct {
+	attempts int
+	// Initial value to scale out
+	Initial time.Duration
+	// Jitter value randomizes an additional delay between 0 and Jitter
+	Jitter time.Duration
+	// Max maximum values of the backoff
+	Max time.Duration
+}
+
+// Returns the current value of the counter and then multiplies it
+// Factor
+func (b *Backoff) Duration() (dur time.Duration) {
+	// Zero-values are nonsensical, so we use
+	// them to apply defaults
+	if b.Max == 0 {
+		b.Max = 10 * time.Second
+	}
+
+	if b.Initial == 0 {
+		b.Initial = 100 * time.Millisecond
+	}
+
+	// calculate this duration
+	if dur = time.Duration(1 << uint(b.attempts)); dur > 0 {
+		dur = dur * b.Initial
+	} else {
+		dur = b.Max
+	}
+
+	if b.Jitter > 0 {
+		dur = dur + time.Duration(rand.Intn(int(b.Jitter)))
+	}
+
+	// bump attempts count
+	b.attempts++
+
+	return dur
+}
+
+//Resets the current value of the counter back to Min
+func (b *Backoff) Reset() {
+	b.attempts = 0
+}
+
+// Attempts returns the number of attempts that we had done so far
+func (b *Backoff) Attempts() int {
+	return b.attempts
+}