boost.http private

This is the initial required code. There is still much to be done. TODO: * Manual functor classes (i.e. not using lambda expressions) were used everywhere to ease up diff-reviewing, but for a few small functions, lambda expressions would work better to specify the initiation object. I'm probably never gonna fix this TODO. * Only `basic_socket<T>` was converted. There is still many other classes that need to be converted to use `async_initiate`. * `file_server.hpp` should be converted to use the new `async_compose` facility. However, the code already compiles under Boost 1.70 and I don't want to delay this small delivery much further as I have other projects to focus on.