Future<T>::then Future<T>::then() -> Future<T>::thenValue or thenTry.
Future<T>::then Future<T>::then() -> Future<T>::thenValue or thenTry.

Summary:
Part of the larger project 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.

Codemod:
  future<T>.then(callable with operator()(not-a-try)) to future<T>.thenValue(callable with operator()(not-a-try)).
  future<T>.then(callable with operator()()) to future<T>.thenValue(callable with operator()(auto&&)).
  future<T>.then(callable with operator()(auto)) to future<T>.thenValue(callable with operator()(auto)).
  future<T>.then(callable with operator()(folly::Try<T>)) to future<T>.thenTry(callable)

Reviewed By: Orvid

Differential Revision: D10859128

fbshipit-source-id: 6df42d7d9bee324a118c114ff7ada76f93d94268

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly