automatic throttle burst size, #24699 (#24784)

* automatic throttle burst size, #24699

* throttleEven used maxBurst=Int.MaxValue, which in practise resulted
  in no throttling at all at high rates
* The original problem that throttleEven tried to solve was that when
  the throttle rate (cost / per) was high the cost of scheduling
  dominated and became much lower than the given rate, if 0 or low
  maxBurst was given. Difficult for user to know what maxBurst to use.
* In fact, that was already the case for rates > 30/s.
* This is fixed by automatically adjusting the maxBurst for higher
  throttle rates in a new throttle overload that doesn't include
  maxBurst parameter
* Also skipped the mode parameter for that variant since Shaping is
  what almost always is what you want, and otherwise you can use
  the full signature with a good maxBurst
* Deprecated throttleEven, since it is the same. Also fixed the
  implementation of throttleEven to use the automatic burst size,
  since Int.MaxValue is not useful at all.

akka-docs/src/test/scala/docs/stream/QuickStartDocSpec.scala

@@ -62,7 +62,7 @@ class QuickStartDocSpec extends WordSpec with BeforeAndAfterAll with ScalaFuture
     //#add-streams
     factorials
       .zipWith(Source(0 to 100))((num, idx) ⇒ s"$idx! = $num")
-      .throttle(1, 1.second, 1, ThrottleMode.shaping)
+      .throttle(1, 1.second)
       //#add-streams
       .take(3)
       //#add-streams