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:

iOS/FlipperKit/FlipperClient.mm

@@ -124,6 +124,7 @@ using WrapperPlugin = facebook::flipper::FlipperCppWrapperPlugin;
   _cppClient->start();
 }
 
+
 - (void)stop
 {
   _cppClient->stop();

iOS/FlipperKit/FlipperPlugin.h

@@ -36,4 +36,10 @@ longer valid to use.
 */
 - (void)didDisconnect;
 
+/**
+ Returns true if the plugin is meant to be run in background too, otherwise it returns false.
+ */
+@optional
+- (BOOL)runInBackground;
+
 @end