Cancel the timeout in timed_wait on success

Summary:
`timed_wait` is not expected to frequently time-out, so by canceling the timer we can save a wake-up of the future timekeeper eventbase.

In reality we don't always save it because in order to cancel the timeout we have to enqueue a task in the eventbase, which also may wake it up, unless the queue is already non-empty in which case we can enjoy some batching, but on average this should be a minor improvement (and we may optimize that separately).

Other timeout primitives like `folly::coro::timeout` already do this.

Reviewed By: dmm-fb

Differential Revision: D48832670

fbshipit-source-id: c3534aa067b7e49b44954b5f81938754e2d6eefd

folly/experimental/coro/TimedWait.h

@@ -40,11 +40,12 @@ timed_wait(Awaitable awaitable, Duration duration) {
       detail::lift_lvalue_reference_t<semi_await_result_t<Awaitable>>>>>
       result;
 
+  Executor* executor = co_await co_current_executor;
   auto sleepFuture = futures::sleep(duration).toUnsafeFuture();
   auto posted = new std::atomic<bool>(false);
-  std::move(sleepFuture)
-      .setCallback_([posted, &baton, executor = co_await co_current_executor](
-                        auto&&, auto&&) {
+  sleepFuture.setCallback_(
+      [posted, &baton, executor = Executor::KeepAlive<>{executor}](
+          auto&&, auto&&) {
         if (!posted->exchange(true, std::memory_order_acq_rel)) {
           executor->add([&baton] { baton.post(); });
         } else {
@@ -58,9 +59,10 @@ timed_wait(Awaitable awaitable, Duration duration) {
              awaitable)]() mutable -> Task<semi_await_result_t<Awaitable>> {
           co_return co_await std::move(awaitable);
         });
-    std::move(t)
-        .scheduleOn(co_await co_current_executor)
-        .start([posted, &baton, &result](auto&& r) {
+    std::move(t).scheduleOn(executor).start(
+        [posted, &baton, &result, sleepFuture = std::move(sleepFuture)](
+            auto&& r) mutable {
+          sleepFuture.cancel();
           if (!posted->exchange(true, std::memory_order_acq_rel)) {
             result = std::move(r);
             baton.post();