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

iOS/FlipperKit/CppBridge/FlipperCppWrapperPlugin.h

@@ -38,6 +38,13 @@ public:
 
   void didDisconnect() override { [_objCPlugin didDisconnect]; }
 
+  bool runInBackground() override {
+    if ([_objCPlugin respondsToSelector:@selector(runInBackground)]) {
+      return [_objCPlugin runInBackground];
+    }
+    return false;
+  }
+
   ObjCPlugin getObjCPlugin() { return _objCPlugin; }
 
 private: