Switch RocketClient to use the FrozenOptions getFiberManager API

Summary: Otherwise its possible to hit a collision with another code that's using unsafe getFiberManager API.

Reviewed By: yfeldblum

Differential Revision: D21492532

fbshipit-source-id: 52577a284f80e4d7b7e730adde24812e53702c29

folly/fibers/FiberManagerInternal.h

@@ -422,6 +422,10 @@ class FiberManager : public ::folly::Executor {
   static FiberManager& getFiberManager();
   static FiberManager* getFiberManagerUnsafe();
 
+  const Options& getOptions() const {
+    return options_;
+  }
+
  private:
   friend class Baton;
   friend class Fiber;