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

test/http/header_parser.cpp

@@ -0,0 +1,66 @@
+//
+// Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+// Test that header file is self-contained.
+#include <beast/http/header_parser.hpp>
+
+#include <beast/core/flat_streambuf.hpp>
+#include <beast/http/read.hpp>
+#include <beast/unit_test/suite.hpp>
+#include <beast/test/string_istream.hpp>
+#include <beast/test/yield_to.hpp>
+
+namespace beast {
+namespace http {
+
+class header_parser_test
+    : public beast::unit_test::suite
+    , public test::enable_yield_to
+{
+public:
+    void
+    testParse()
+    {
+        {
+            test::string_istream is{ios_,
+                "GET / HTTP/1.1\r\n"
+                "User-Agent: test\r\n"
+                "\r\n"
+            };
+            flat_streambuf db{1024};
+            header_parser<true, fields> p;
+            read_some(is, db, p);
+            BEAST_EXPECT(p.is_complete());
+        }
+        {
+            test::string_istream is{ios_,
+                "POST / HTTP/1.1\r\n"
+                "User-Agent: test\r\n"
+                "Content-Length: 1\r\n"
+                "\r\n"
+                "*"
+            };
+            flat_streambuf db{1024};
+            header_parser<true, fields> p;
+            read_some(is, db, p);
+            BEAST_EXPECT(! p.is_complete());
+            BEAST_EXPECT(p.state() == parse_state::body);
+        }
+    }
+
+    void
+    run() override
+    {
+        testParse();
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(header_parser,http,beast);
+
+} // http
+} // beast
+