[core] Make the BackgroundScheduler a singleton

- Do not carry it over everywhere as parameter, it is a shared
  instance anyway and the lifecycle is pretty much the app lifecycle
  from the moment we instantiate a map.
- Rename to BackgroundScheduler because it is a Scheduler that will
  do tasks in the background, we don't make assumptions if it is a
  thread pool or a single thread.
- Most importantly, remove the dependency from `core` on `platform`.

platform/node/src/node_map.hpp

@@ -1,7 +1,5 @@
 #pragma once
 
-#include "node_thread_pool.hpp"
-
 #include <mbgl/map/map.hpp>
 #include <mbgl/storage/file_source.hpp>
 #include <mbgl/util/image.hpp>
@@ -79,7 +77,6 @@ public:
     const float pixelRatio;
     mbgl::MapMode mode;
     bool crossSourceCollisions;
-    NodeThreadPool threadpool;
     NodeMapObserver mapObserver;
     std::unique_ptr<mbgl::HeadlessFrontend> frontend;
     std::unique_ptr<mbgl::Map> map;