fix for TSAN-reported data-race in coro::timed_wait

Summary:
TSAN aborts on a data-race of the form:
```
WARNING: ThreadSanitizer: data race (pid=978596)
  Write of size 8 at 0x7b0400000b40 by thread T2:
    #0 operator delete(void*, unsigned long) <null>
    #1 folly::coro::timed_wait</*...*/>(/*...*/)::'lambda'(/*...*/)::operator()</*...*/>(/*...*/) const folly/experimental/coro/TimedWait.h:51
  Previous atomic write of size 1 at 0x7b0400000b40 by thread T1:
    #0 __tsan_atomic8_exchange <null>
    #1 std::atomic<bool>::exchange(bool, std::memory_order) libgcc/include/c++/bits/atomic_base.h:499
    #2 folly::coro::timed_wait</*...*/>(/*...*/)::'lambda'(/*...*/)::operator()</*...*/>(/*...*/) const folly/experimental/coro/TimedWait.h:64
```

Reviewed By: davidtgoldblatt, iahs

Differential Revision: D36159450

fbshipit-source-id: 860fcbbc1f689f9e166934f75a2ef60abdcad421

folly/experimental/coro/TimedWait.h

@@ -45,7 +45,7 @@ timed_wait(Awaitable awaitable, Duration duration) {
   std::move(sleepFuture)
       .setCallback_([posted, &baton, executor = co_await co_current_executor](
                         auto&&, auto&&) {
-        if (!posted->exchange(true, std::memory_order_relaxed)) {
+        if (!posted->exchange(true, std::memory_order_acq_rel)) {
           executor->add([&baton] { baton.post(); });
         } else {
           delete posted;
@@ -61,7 +61,7 @@ timed_wait(Awaitable awaitable, Duration duration) {
     std::move(t)
         .scheduleOn(co_await co_current_executor)
         .start([posted, &baton, &result](auto&& r) {
-          if (!posted->exchange(true, std::memory_order_relaxed)) {
+          if (!posted->exchange(true, std::memory_order_acq_rel)) {
             result = std::move(r);
             baton.post();
           } else {