Should Http.Proto support `-fno-rtti`?

[cmazakas]

The service registry has a dependency on type_index which requires RTTI, so the code currently polyfills this via: https://github.com/cppalliance/http_proto/blob/develop/include/boost/http_proto/detail/type_index.hpp

This incurs extra maintenance burden not to mention the introduction of a downcast helper, which was created to facilitate the use-case where a derived service object uses virtual inheritance.

It'd be good to compare our implementation against the one in Asio, which seems to only return void* in the case of -fno-rtti.