Refactoring beast::asio:
* New tools for completion handlers:
  - shared_handler manages any completion handler.
  - make_wrapped provides io_service execution guarantees.
* Removed obsolete classes and functions
  - AbstractHandler
  - ComposedAsyncOperation
  - SharedFunction
  - SharedHandler
  - SharedHandlerAllocator
  - SharedHandlerPtr
  - SharedHandlerType
  - wrapHandler