[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/Network/XMLHttpRequest.ios.js

@@ -89,28 +89,24 @@ class XMLHttpRequest extends XMLHttpRequestBase {
     if (data instanceof FormData) {
       data = {formData: data.getParts()};
     }
-    RCTDataManager.queryData(
-      'http',
+    RCTDataManager.sendRequest(
       {
         method,
         url,
         data,
         headers,
+        incrementalUpdates: this.onreadystatechange ? true : false,
       },
-      this.onreadystatechange ? true : false,
       this._didCreateRequest.bind(this)
     );
   }
 
   abortImpl(): void {
     if (this._requestId) {
+      RCTDataManager.cancelRequest(this._requestId);
       this._clearSubscriptions();
       this._requestId = null;
     }
-    console.warn(
-      'XMLHttpRequest: abort() cancels JS callbacks ' +
-      'but not native HTTP request.'
-    );
   }
 }