Drop some uses of std::static_pointer_cast
Drop some uses of std::static_pointer_cast

Summary:
These were necessary to make std::unordered_map<K,V>::emplace<Args&&...>() infer
the correct constructor to call for the value type.  A shorter way to do this is
to construct the value type directly, and pass it in as V&&.

Reviewed By: phoad

Differential Revision: D8001988

fbshipit-source-id: 3a63e97edf028edccf3b07c40f7739bcdd85cd0e

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