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/streambuf_body.cpp

@@ -10,7 +10,7 @@
 
 #include <beast/core/to_string.hpp>
 #include <beast/http/fields.hpp>
-#include <beast/http/parser_v1.hpp>
+#include <beast/http/message_parser.hpp>
 #include <beast/http/read.hpp>
 #include <beast/http/write.hpp>
 #include <beast/test/string_istream.hpp>
@@ -34,11 +34,12 @@ public:
             "\r\n"
             "xyz";
         test::string_istream ss(ios_, s);
-        parser_v1<false, streambuf_body, fields> p;
+        message_parser<false, streambuf_body, fields> p;
         streambuf sb;
-        parse(ss, sb, p);
-        BEAST_EXPECT(to_string(p.get().body.data()) == "xyz");
-        BEAST_EXPECT(boost::lexical_cast<std::string>(p.get()) == s);
+        read(ss, sb, p);
+        auto const& m = p.get();
+        BEAST_EXPECT(to_string(m.body.data()) == "xyz");
+        BEAST_EXPECT(boost::lexical_cast<std::string>(m) == s);
     }
 };