Move DevTools integration into its own repo

Summary:
The way React DevTools integration was set up in RN was not entirely supported by the React team, and we had to disable it in c3b25c9059f57ee8df5505628ff6221f11cf9234 so that we can move forward with enabling Fiber support in React Native.

Here, I am moving the DevTools client setup from RN repo to [React DevTools repo](https://github.com/facebook/react-devtools/blob/master/packages/react-devtools-core/README.md#requirereact-devtools-coreconnecttodevtoolsoptions) so that we can keep it in sync with the rest of React DevTools. This is also a part of a larger effort to consolidate DevTools code (facebook/react-devtools#489). It allows us to remove the double injection of the hook, an lets us replace the `eval` hack with a regular dependency. The implementation [lives here now](https://github.com/facebook/react-devtools/blob/master/packages/react-devtools-core/src/backend.js).

This change re-enables Nuclide Inspector with React Native Stack reconciler and prepares it for compatibi
Closes facebook/react-native#12316

Reviewed By: zertosh

Differential Revision: D4545322

Pulled By: gaearon

fbshipit-source-id: ab949916c1a92c6b41cd41e7e1edf9697a71de2e

Author: gaearon

server/runServer.js

@@ -69,7 +69,6 @@ function runServer(args, config, readyCallback) {
 
       wsProxy = webSocketProxy.attachToServer(serverInstance, '/debugger-proxy');
       ms = messageSocket.attachToServer(serverInstance, '/message');
-      webSocketProxy.attachToServer(serverInstance, '/devtools');
       inspectorProxy.attachToServer(serverInstance, '/inspector');
       readyCallback();
     }