periodic timers should not cause leaking of folly::RequestContext

Summary:
we do have some periodic maintentenance tasks... if those are initialized
lazily on a requests handling path, they  will capture current rctx and keep it
it alive forever.

Reviewed By: yfeldblum

Differential Revision: D22859344

fbshipit-source-id: 3655d874dee22aaf74b4d80631d4a58e83539752

folly/fibers/FiberManager.cpp

@@ -200,6 +200,7 @@ void FiberManager::doFibersPoolResizing() {
 void FiberManager::FibersPoolResizer::run() {
   fiberManager_.doFibersPoolResizing();
   if (auto timer = fiberManager_.loopController_->timer()) {
+    RequestContextScopeGuard rctxGuard(std::shared_ptr<RequestContext>{});
     timer->scheduleTimeout(
         this,
         std::chrono::milliseconds(