Use std::mutex instead of never-shared std::shared_mutex in ShadowNodeFamily

ShadowNodeFamily::mutex_ is declared std::shared_mutex but has not been
acquired in shared mode since February 2020. All three call sites take it
exclusively via unique_lock, and there is no shared_lock in the class.

Of the 17 classes in ReactCommon that declare a std::shared_mutex, this is
the only one that never takes a shared_lock.

Reduces sizeof(ShadowNodeFamily) from 384 to 272 bytes on arm64 (-112, -29%).
There is one family per host element, so this scales with tree size.

Changelog: [GENERAL] [CHANGED] - Replace never-shared std::shared_mutex with std::mutex in ShadowNodeFamily

Author: amillez

packages/react-native/ReactCommon/react/renderer/core/ShadowNodeFamily.h

@@ -9,7 +9,6 @@
 
 #include <memory>
 #include <mutex>
-#include <shared_mutex>
 
 #include <react/renderer/core/EventEmitter.h>
 #include <react/renderer/core/InstanceHandle.h>
@@ -145,7 +144,7 @@ class ShadowNodeFamily final : public jsi::NativeState {
 
   EventDispatcher::Weak eventDispatcher_;
   std::shared_ptr<const State> mostRecentState_;
-  mutable std::shared_mutex mutex_;
+  mutable std::mutex mutex_;
 
   std::function<void(ShadowNodeFamily &family)> onUnmountedFamilyDestroyedCallback_ = nullptr;