Setup sdk for background plugin

Summary:
This diff sets up flipper for running plugins in background. This diff does the following

- Adds a function named `runInBackground` to the interface `FlipperPlugin` to make the plugins opt in to be run in background, default is false
- Changes the javascript side of the flipper to store the messages received by the plugins in background
- Process the stored messages when the plugin in background becomes active
- Currently I have just turned on network plugin to be in background mode.

- Remove the buffering from the network plugin, as it will run in background
- Write a batching layer to batch the messages and send to flipper.

Note: I haven't tested the wilde app yet, but the sample app works. I will remove the "[WIP]" from the title once I have tested it in wilde

Reviewed By: danielbuechele

Differential Revision: D10301403

fbshipit-source-id: 034eebf659a545d6b480a4ac1b73b0aa4b2f9797

src/dispatcher/server.js

@@ -13,6 +13,12 @@ import type Client from '../Client.js';
 
 export default (store: Store, logger: Logger) => {
   const server = new Server(logger);
+  store.subscribe(() => {
+    let currentState = store.getState();
+    currentState.connections.clients.forEach((client: Client) => {
+      client.selectedPlugin = currentState.connections.selectedPlugin;
+    });
+  });
   server.addListener('new-client', (client: Client) => {
     store.dispatch({
       type: 'NEW_CLIENT',