[core] Add a pause/resume API to the DefaultFileSource

It will halt the networking thread completely. Expired and error'ed
tiles won't get re-requested until resume() is called again.

include/mbgl/storage/default_file_source.hpp

@@ -115,6 +115,9 @@ public:
      */
     void setOfflineMapboxTileCountLimit(uint64_t) const;
 
+    void pause();
+    void resume();
+
     // For testing only.
     void put(const Resource&, const Response&);