Cache local images into memory (microsoft#1279)

This introduces a cache for local image on macOS. There are no predefined limits set here (at least yet) since presumably we are loading a finite number of resources and we have the benefit of virtual memory swap on desktop so we don't really need to sweat a few extra megabytes of in-memory cache for images we know we'll need to repeatedly load.

Local images don't use the shared RN image cache, and on macOS there's no automatic in-memory cache for local resources (other than the OS-level disk cache) so the `RCTImageFromLocalBundleAssetURL` function will synchronously access the disk on the main thread many times over during thread switching in Messenger Desktop.

Added logging to confirmed cache works as expected in rn-tester, and instrumentation also confirms it.

Co-authored-by: Scott Kyle <skyle@fb.com>

React/Base/RCTUtils.m

@@ -21,6 +21,8 @@
 #import "RCTAssert.h"
 #import "RCTLog.h"
 
+static const NSUInteger RCTMaxCachableImageCount = 100;
+
 NSString *const RCTErrorUnspecified = @"EUNSPECIFIED";
 
 // Returns the Path of Home directory