Enforce r-value use of Future::getVia

Summary:
Fixes across the codebase to allow for r-value qualified getVia.

Change is behaviour neutral - getVia is already destructive internally.

Reviewed By: yfeldblum

Differential Revision: D23605722

fbshipit-source-id: 75dfe6faca1c888eae9b262552372dd557eb6933

folly/executors/test/ExecutorTest.cpp

@@ -181,7 +181,7 @@ TEST(ManualExecutor, getViaDoesNotDeadlock) {
                .via(&west);
   std::thread t([&] {
     baton.post();
-    f.getVia(&west);
+    std::move(f).getVia(&west);
   });
   baton.wait();
   east.run();