[ReactNative] Add RCTAssertThread and restrict -[UIManager addUIBlock:] to _shadowQueue

Summary:
@public

Add `RCTAssertThread` to `RCTAssert.h` for convenience when checking the current/queue,
it accepts either a `NSString *`, `NSThread *` or `dispatch_queue_t` as the object to be checked

Also add a check to `-[RCTUIManager addUIBlock:]` - There was a discussion on github (https://github.com/facebook/react-native/issues/1365)
due to the weird behavior caused by calling it from a different thread/queue (it might be added after `batchDidComplete` has been called
and will just be dispatched on the next call from JS to objc)

Test Plan:
Change `-[RCTAnimationExperimentalManager methodQueue]` to return `dispatch_get_main_queue()` and run the 2048 example,
it should dispatch with a helpful message (screenshot on the comments)

React/Base/RCTAssert.m

@@ -60,3 +60,20 @@ void RCTAddAssertFunction(RCTAssertFunction assertFunction)
     RCTCurrentAssertFunction = assertFunction;
   }
 }
+
+NSString *RCTCurrentThreadName(void)
+{
+  NSThread *thread = [NSThread currentThread];
+  NSString *threadName = [thread isMainThread] ? @"main" : thread.name;
+  if (threadName.length == 0) {
+#if DEBUG // This is DEBUG not RCT_DEBUG because it *really* must not ship in RC
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+    threadName = @(dispatch_queue_get_label(dispatch_get_current_queue()));
+#pragma clang diagnostic pop
+#else
+    threadName = [NSString stringWithFormat:@"%p", thread];
+#endif
+  }
+  return threadName;
+}