Fix dev-tsan error

Summary:
TSAN catches this as a data race, though it's fairly benign. In the
assert we read non-atomic `state` without holding the lock. As a fix, acquire
the lock in the assert. The assert is skipped anyway in opt builds.

Reviewed By: yfeldblum

Differential Revision: D28302491

fbshipit-source-id: b92ca9f3d321d34301d96e59fab363d999bddb30

folly/fibers/TimedMutex-inl.h

@@ -173,7 +173,10 @@ void TimedRWMutexImpl<ReaderPriority, BatonType>::lock_shared() {
     read_waiters_.push_back(waiter);
     ulock.unlock();
     waiter.baton.wait();
+    if (folly::kIsDebug) {
+      std::unique_lock<folly::SpinLock> assertLock{lock_};
       assert(state_ == State::READ_LOCKED);
+    }
     return;
   }
   assert(