Synchronously render cached images

Summary:
## Problem
Previously the RN Image render the cached images asynchronously (line 555 and 594 prior to the change), which caused the images to render at least a frame later than the adjacent components.

## Change
In this change, we call partialLoadHandler with the cached image synchronously on the main thread.

Changelog: [iOS][Changed] - Synchronously render cached images

Reviewed By: p-sun

Differential Revision: D34487673

fbshipit-source-id: 0600c2fa5f7a1eca71b8582bbe968694cf211468

Libraries/Image/RCTImageLoader.mm

@@ -492,6 +492,16 @@ - (RCTImageURLLoaderRequest *)_loadImageOrDataWithURLRequest:(NSURLRequest *)req
   BOOL cacheResult = [loadHandler respondsToSelector:@selector(shouldCacheLoadedImages)] ?
   [loadHandler shouldCacheLoadedImages] : YES;
 
+  if (cacheResult) {
+    UIImage *image = [[self imageCache] imageForUrl:request.URL.absoluteString
+                                               size:size
+                                              scale:scale
+                                         resizeMode:resizeMode];
+    if (image) {
+      partialLoadHandler(image);
+    }
+  }
+
   auto cancelled = std::make_shared<std::atomic<int>>(0);
   __block dispatch_block_t cancelLoad = nil;
   __block NSLock *cancelLoadLock = [NSLock new];