(wangle) ManualExecutor::waitFor(F&&)

Summary:
@override-unit-failures

Test Plan: Using it in D1322350

Reviewed By: hannesr@fb.com

FB internal diff: D1322475

folly/wangle/ManualExecutor.h

@@ -42,6 +42,11 @@ namespace folly { namespace wangle {
       run();
     }
 
+    template <class F> void waitFor(F const& f) {
+      while (!f.isReady())
+        makeProgress();
+    }
+
    private:
     std::mutex lock_;
     std::queue<std::function<void()>> runnables_;