Name the Flipper scheduler/threads

Summary:
Plumbs through an optional name for FollyScopedThreadScheduler.
This allows the threads to be named in XCode/lldb.

Differential Revision: D38582449

fbshipit-source-id: 1de50d25c0f91e7003cf81cb22faf4b10a8e23a8

xplat/Flipper/FlipperFollyScopedThreadScheduler.h

@@ -17,6 +17,10 @@ namespace facebook {
 namespace flipper {
 
 struct FollyScopedThreadScheduler : public Scheduler {
+  FollyScopedThreadScheduler() : FollyScopedThreadScheduler("") {}
+  FollyScopedThreadScheduler(folly::StringPiece name)
+      : thread_(nullptr, name) {}
+
   virtual void schedule(Func&& t) override {
     thread_.getEventBase()->add(t);
   }