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

xplat/Flipper/FlipperConnectionManagerImpl.cpp

@@ -92,7 +92,7 @@ void FlipperConnectionManagerImpl::start() {
   folly::makeFuture()
       .via(flipperEventBase_->getEventBase())
       .delayed(std::chrono::milliseconds(0))
-      .thenValue([this, step](auto&&){ step->complete(); startSync(); });
+  .thenValue([this, step](auto&&){ step->complete(); startSync();});
 }
 
 void FlipperConnectionManagerImpl::startSync() {