Future<T>::then 6/n: Future<T>::then() & -> Future<T>::then() &&.
Future<T>::then 6/n: Future<T>::then() & -> Future<T>::then() &&.

Summary:
Overall plan to modify Future<T>::then to be r-value qualified and use Future<T>::thenTry or Future<T>::thenValue.

The goal is to disambiguate folly::Future and to improve type and lifetime safety of Future and its methods.

6/n: Codemod rvalue-future<T>.then(...) to rvalue-future<T>.then(...).

Reviewed By: yfeldblum

Differential Revision: D9152104

fbshipit-source-id: 161906bfd1db06ecf84989d0a0e9bd23ee365185

rsocket/rsocket-cpp,phoad/rsocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,phoad/rsocket-cpp,phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp