[ReactNative] Refactor RCTDataManager to support pluggable data source modules (RCTURLRequestHandlers)

Summary:
@public

This is a refactor of @philikon's original diff that decouples the dependencies between the Network and Image modules, and replaces RCTDataQueryExecutor with a more useful abstraction.

I've introduced the RCTURLRequestHandler protocol, which is a new type of bridge module used for loading data using an NSURLRequest. RCTURLRequestHandlers can be registered using RCT_EXPORT_MODULE() and are then available at runtime for use by the RCTDataManager, which will automatically select the appropriate handler for a given request based on the handler's self-reported capabilities.

The currently implemented handlers are:

- RCTHTTPRequestHandler - the standard open source HTTP request handler that uses NSURLSession
- RKHTTPRequestHandler - the internal FB HTTP request handler that uses FBNetworking
- RCTImageRequestHandler - a handler for loading local images from the iOS asset-library

Depends on D2108193

Test Plan:
- Internal apps still work
- OSS port still compiles, Movies app and a sample Parse app still work
- uploading image to Parse using the above code snippet works
- tested `FormData` with string and image parameters using http://www.posttestserver.com/

Libraries/Image/RCTImageLoader.m

@@ -16,11 +16,23 @@
 #import <UIKit/UIKit.h>
 
 #import "RCTConvert.h"
+#import "RCTDefines.h"
 #import "RCTGIFImage.h"
 #import "RCTImageDownloader.h"
 #import "RCTLog.h"
 #import "RCTUtils.h"
 
+static void RCTDispatchCallbackOnMainQueue(void (^ __nonnull callback)(NSError *, id), NSError *error, UIImage *image)
+{
+  if ([NSThread isMainThread]) {
+    callback(error, image);
+  } else {
+    dispatch_async(dispatch_get_main_queue(), ^{
+      callback(error, image);
+    });
+  }
+}
+
 static dispatch_queue_t RCTImageLoaderQueue(void)
 {
   static dispatch_queue_t queue = NULL;
@@ -137,11 +149,4 @@ static dispatch_queue_t RCTImageLoaderQueue(void)
   }
 }
 
-+ (BOOL)isSystemImageURI:(NSString *)uri
-{
-  return uri != nil && (
-   [uri hasPrefix:@"assets-library"] ||
-   [uri hasPrefix:@"ph://"]);
-}
-
 @end