Enable request timeout callback to be any HHWheelTimer callback

Summary: This diff factors out some of the logic needed in the stacked diff D21628842. We want `RocketClientChannel::sendRequestResponse` to work without a fiber if the caller is not already on a fiber, which means we cannot only use `Baton::TimeoutHandler` for the request's timeout callback.

Reviewed By: iahs

Differential Revision: D21628871

fbshipit-source-id: 2e739f205fcb0a255108fa88f94107378a1bab49

folly/fibers/Baton.h

@@ -216,7 +216,7 @@ class Baton {
    * scheduleTimeout() may only be called once prior to the end of the
    * associated Baton's life.
    */
-  class TimeoutHandler final : private HHWheelTimer::Callback {
+  class TimeoutHandler final : public HHWheelTimer::Callback {
    public:
     void scheduleTimeout(std::chrono::milliseconds timeout);