Add beast::asio::basic_streambuf (RIPD-661):
This is class whose interface is identical to the boost::asio::basic_streambuf,
and uses an implementation that stores the data in multiple discontiguous
linear buffers, expanding and shrinking as needed.
Use beast::asio::streambuf in Overlay
Separate beast::http::body from beast::http::message (RIPD-660):
This changes the http::message object to no longer contain a body. It modifies
the parser to store the body in a separate object, or to pass the body data
to a functor. This allows the body to be stored in more flexible ways. For
example, in HTTP responses the body can be generated procedurally instead
of being required to exist entirely in memory at once.