Phase out Promise::getFuture in rsocket.
Phase out Promise::getFuture in rsocket.

Summary: folly::Promise::getFuture is deprecated in favour of getSemiFuture to avoid inline execution of callbacks. This cleans fbcode/rsocket/* to remove such instances.

Reviewed By: phoad

Differential Revision: D6930875

fbshipit-source-id: f49251dd35ce3469788c487f376d13e9d4184ee3

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