fix#1375
* The value returned from `basic_parser::content_length`
no longer changes as the body of the message is received.
Actions Required:
* Call `basic_parser::content_length_remaining` instead
of `basic_parser::content_length`.
* `basic_parser` now uses pure virtual member functions instead
of the curiously recurring template pattern.
Actions Required:
* Change uses of the `basic_parser` type to omit the `Derived`
template parameter
* Classes derived from `basic_parser` no longer need to friend
the base.
* Virtual functions in the derived class may be marked `override`.
Some `basic_parser` member functions are renamed:
* chunked() was is_chunked()
* upgrade() was is_upgrade()
* keep_alive() was is_keep_alive()
Actions Required:
* Rename member function invocations at call sites
fix#769
The following classes are removed:
* handler_type
* async_result
* async_completion
* is_dynamic_buffer
* is_const_buffer_sequence
* is_mutable_buffer_sequence
* handler_alloc
Actions Required:
* Use BOOST_ASIO_HANDLER_TYPE instead of handler_type
* Use BOOST_ASIO_INITFN_RESULT_TYPE instead of async_result
* Use boost::asio::async_completion
* Use boost::asio::is_dynamic_buffer
* Use boost::asio::is_const_buffer_sequence
* Use boost::asio::is_mutable_buffer_sequence
* boost::asio::associated_allocator_t replaces handler_alloc
