New HTTP interfaces (API Change):

fix #123
fix #154
fix #265

This completely replaces the HTTP parser used to read and
parse incoming messages. The new version is faster than
the old one, and written in fewer lines. A summary of
changes:

* parse and async_parse renamed to read and async_read

* basic_parser is optimized for linear buffers,
  use flat_streambuf for the best performance with these
  functions:

  - http::read
  - http::read_some
  - http::async_read
  - http::async_read_some

* The overloads of read() and async_read() which take
  just a header have been removed, since they would
  throw away important parse metadata.

* The derived class callbacks for basic_parser have
  been streamlined. All strings passed to callbacks
  are presented in their entirety, instead of being
  provided in pieces.

These changes allow use-cases that were previously
difficult or impossible, such as:

- Efficient relaying

- Late body type commitment

- Expect: 100-continue handling

examples/http_example.cpp

@@ -22,7 +22,7 @@ int main()
         r.resolve(boost::asio::ip::tcp::resolver::query{host, "http"}));
 
     // Send HTTP request using beast
-    beast::http::request<beast::http::empty_body> req;
+    beast::http::request<beast::http::string_body> req;
     req.method = "GET";
     req.url = "/";
     req.version = 11;