Remove .then from FutureSplitter

Summary: To allow deprecation, remove use cases of Future::then inside FutureSplitter.h

Reviewed By: yfeldblum

Differential Revision: D9995264

fbshipit-source-id: d44c63c45a08ca675b9bff33fabcbb2836ea2a64

folly/futures/FutureSplitter.h

@@ -53,7 +53,7 @@ class FutureSplitter {
       : promise_(std::make_shared<SharedPromise<T>>()),
         e_(getExecutorFrom(future)),
         priority_(future.getPriority()) {
-    std::move(future).then([promise = promise_](Try<T>&& theTry) {
+    std::move(future).thenTry([promise = promise_](Try<T>&& theTry) {
       promise->setTry(std::move(theTry));
     });
   }