Allow native modules to customize the main-thread init behaviour

Summary:
We've simplified a lot of the conditions for eager of the module init so now we can introduce a final switch to allow modules to opt-out (and in the future opt-in if they still require the behaviour).

We now require you to be explicit about the intended behaviour and implement the `+ (BOOL)requiresMainQueueSetup` method on your module. When you return YES from this method, it tells the bridge the module needs to be created on the main thread (and to avoid deadlocks, we do so eagerly during bridge startup). When you return NO, the native module will be initialised when it's first accessed from JS.

The current behaviour is maintained but a warning is emitted until the new API is adopted.

Reviewed By: fkgozali

Differential Revision: D5527788

fbshipit-source-id: 56d38f81e58cf950547b9780e89bfac4667eeaaa

React/CxxModule/RCTCxxModule.mm

@@ -28,6 +28,11 @@ using namespace facebook::react;
   return @"";
 }
 
++ (BOOL)requiresMainQueueSetup
+{
+  return NO;
+}
+
 - (void)lazyInit
 {
   if (!_module) {