Fix data race in SingletonHolder<T>::hasLiveInstance

Summary:
hasLiveInstance() accesses instance_weak_ which is not safe to access concurrently with createInstance().
In all other places hasLiveInstance() is already called while holding the vault state lock or after switching state to shutdown (which prevents concurrent creation).

livingSingletonCount() should be performing similar synchronization as well.

Reviewed By: dsesh

Differential Revision: D62614961

fbshipit-source-id: 7552e46fc12fb4e6e02c5094afb1d4a4e0b7804c

folly/Singleton.h

@@ -497,6 +497,7 @@ class SingletonVault {
   bool eagerInitComplete() const;
 
   size_t livingSingletonCount() const {
+    auto state = state_.rlock();
     auto singletons = singletons_.rlock();
 
     size_t ret = 0;