[Finagle] Backoff.takeUntil doesn't call the underlying backoff's next()

Problem:

When a new `c.t.f.Backoff`  is created by wrapping an existing instance with `c.t.f.takeUntil` it incorrectly always only returns the underlying Backoff's first backoff

Solution:

Instead it should iterate over the underlying backoff by calling `next()` on it

JIRA Issues: RTC-2278

Differential Revision: https://phabricator.twitter.biz/D1099285

finagle-core/src/main/scala/com/twitter/finagle/Backoff.scala

@@ -359,7 +359,7 @@ object Backoff {
       else Backoff.empty.duration
     def next: Backoff =
       if (count < maxCumulativeBackoff)
-        new TakeWhile(backoff, count + duration, maxCumulativeBackoff)
+        new TakeWhile(backoff.next, count + duration, maxCumulativeBackoff)
       else Backoff.empty.next
     def isExhausted: Boolean = if (count < maxCumulativeBackoff) false else true
   }

finagle-core/src/test/scala/com/twitter/finagle/BackoffTest.scala

@@ -1,7 +1,9 @@
 package com.twitter.finagle
 
 import com.twitter.conversions.DurationOps._
-import com.twitter.finagle.Backoff.{DecorrelatedJittered, EqualJittered, ExponentialJittered}
+import com.twitter.finagle.Backoff.DecorrelatedJittered
+import com.twitter.finagle.Backoff.EqualJittered
+import com.twitter.finagle.Backoff.ExponentialJittered
 import com.twitter.finagle.util.Rng
 import com.twitter.util.Duration
 import org.scalacheck.Gen
@@ -184,11 +186,14 @@ class BackoffTest extends AnyFunSuite with ScalaCheckDrivenPropertyChecks {
   test("takeUntil") {
     var backoff: Backoff = Backoff.linear(1.second, 1.second).takeUntil(9.seconds)
     var sumBackoff: Duration = Duration.Zero
+    val backoffs: ArrayBuffer[Duration] = ArrayBuffer.empty[Duration]
     while (sumBackoff < 9.seconds) {
+      assert(!backoff.isExhausted)
       sumBackoff += backoff.duration
+      backoffs += backoff.duration
       backoff = backoff.next
     }
-    assert(sumBackoff == 9.seconds)
+    assert(backoffs == ArrayBuffer[Duration](1.seconds, 2.seconds, 3.seconds, 4.seconds))
     assert(backoff.isExhausted)
   }