use rvalue-qual Future::semi(); pass 2

Summary:
This is part of "the great r-valuification of folly::Future":
* This is something we should do for safety in general.
* Several of folly::Future's methods are lvalue-qualified even though they act as though they are rvalue-qualified, that is, they provide a postcondition that says, in effect, callers should act as though the method invalidated its `this` object (regardless of whether that invalidation was actual or logical).
* This violates the C++ principle to "Express ideas directly in code" (see Core Guidelines), and generally makes it more confusing for callers as well as hiding the actual semantics from tools (linters, compilers, etc.).
* This dichotomy and confusion has manifested itself by some failures around D7840699 since lvalue-qualification hides that operation's move-out semantics - leads to some use of future operations that are really not correct, but are not obviously incorrect.
* The goal of rvalueification is to make sure methods that are logically rvalue-qualified are actually rvalue-qualified, which forces callsites to acknowledge that rvalueification, e.g., `std::move(f).semi()` instead of `f.semi()`. This syntactic change in the callsites forces callers to acknowledge the method's rvalue semantics.

Reviewed By: LeeHowes

Differential Revision: D9478283

fbshipit-source-id: 7d2f54a8aaa9156b2b274aafc9695fd996f29641

folly/futures/test/FutureTest.cpp

@@ -322,7 +322,7 @@ TEST(Future, hasPostconditionInvalid) {
   DOIT(makeValid(), swallow(std::move(f.wait())));
   DOIT(makeValid(), swallow(std::move(f).get()));
   DOIT(makeValid(), swallow(std::move(f).get(std::chrono::milliseconds(10))));
-  DOIT(makeValid(), swallow(f.semi()));
+  DOIT(makeValid(), swallow(std::move(f).semi()));
 
 #undef DOIT
 }