Add integration with new async/await support in 2.12.12+ and 2.13.3+

[olafurpg]

Problem

Currently, the primary syntax to compose Twitter futures is using map/flatMap syntax. This syntax may feel verbose and unfamiliar to developers who have worked with other languages such as JavaScript or C# that support async/await.

Solution

This PR adds async/await support for com.twitter.util.Future. This functionality is
only available in the newly released 2.13.3 (notes https://github.com/scala/scala/releases/tag/v2.13.3). It will also become available in the upcoming 2.12.12 release.

Result

Users can write code like this now

val c = async {
  val a = Future.value(10)
  val b = Future.value(5)
  await(a) + await(b)
}
assert(c.poll == Some(Return(15)))

The equivalent way to to write the same code above with the current syntax is

  val c = for {
    a <- Future.value(10)
    b <- Future.value(5)
  } yield a + b

Notes

The code in this PR is adapted from the code in scala/scala-async#237

To try out this PR locally run the following commands

$ sbt
> ++2.13.3
> util-core/testOnly com.twitter.util.AsyncTest

[olafurpg]

For the record, I'm not very familiar with Twitter futures or the new -Xasync flag. This PR is just an experiment so I'm converting it into a draft.

[codecov-commenter]

Codecov Report

Merging #279 into develop will increase coverage by 0.62%.
The diff coverage is 64.28%.

@@             Coverage Diff             @@
##           develop     #279      +/-   ##
===========================================
+ Coverage    46.12%   46.75%   +0.62%     
===========================================
  Files          238      238              
  Lines        14414    14063     -351     
  Branches       869      886      +17     
===========================================
- Hits          6649     6575      -74     
+ Misses        7765     7488     -277     

Impacted Files	Coverage Δ
...l-core/src/main/scala/com/twitter/util/Async.scala	0.00% <0.00%> (ø)
...in/scala/com/twitter/util/FutureStateMachine.scala	69.23% <69.23%> (ø)
.../src/main/scala/com/twitter/util/ThriftCodec.scala	80.00% <0.00%> (-5.72%)	⬇️
.../main/scala/com/twitter/util/logging/Logging.scala	26.31% <0.00%> (-1.89%)	⬇️
...twitter/finagle/stats/BroadcastStatsReceiver.scala	49.05% <0.00%> (-1.86%)	⬇️
...r/finagle/stats/NameTranslatingStatsReceiver.scala	87.50% <0.00%> (-1.39%)	⬇️
...l-zk/src/main/scala/com/twitter/zk/Connector.scala	61.29% <0.00%> (-1.21%)	⬇️
...c/main/scala/com/twitter/util/routing/Router.scala	88.88% <0.00%> (-1.12%)	⬇️
...c/main/scala/com/twitter/logging/FileHandler.scala	82.97% <0.00%> (-0.86%)	⬇️
...ore/src/main/scala/com/twitter/util/Duration.scala	84.97% <0.00%> (-0.58%)	⬇️
... and 51 more

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