Remove Promise::getFuture from Future-inl.h 4/4 - window

Summary: Modify folly::window to replace Promise::getFuture calls with Promise::getSemiFuture. The requirement for an executor on the semifuture here meant that I had to modify all of the SemiFutures with an executor in the vector. This may come with a small performance hit in some cases, but appears to be behaviourally better.

Reviewed By: yfeldblum

Differential Revision: D7104920

fbshipit-source-id: 7d105df553c28bb362b37ff64270ee16bf59fbd6

folly/futures/Future-inl.h

@@ -1399,7 +1399,7 @@ window(Executor* executor, Collection input, F func, size_t n) {
   std::vector<Future<Result>> futures;
   futures.reserve(ctx->promises.size());
   for (auto& promise : ctx->promises) {
-    futures.emplace_back(promise.getFuture());
+    futures.emplace_back(promise.getSemiFuture().via(executor));
   }
 
   return futures;