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/message.cpp

@@ -8,7 +8,7 @@
 // Test that header file is self-contained.
 #include <beast/http/message.hpp>
 
-#include <beast/http/empty_body.hpp>
+#include <beast/http/string_body.hpp>
 #include <beast/http/fields.hpp>
 #include <beast/http/string_body.hpp>
 #include <beast/unit_test/suite.hpp>
@@ -195,7 +195,7 @@ public:
     void testFreeFunctions()
     {
         {
-            request<empty_body> m;
+            request<string_body> m;
             m.method = "GET";
             m.url = "/";
             m.version = 11;
@@ -213,7 +213,7 @@ public:
 
     void testPrepare()
     {
-        request<empty_body> m;
+        request<string_body> m;
         m.version = 10;
         BEAST_EXPECT(! is_upgrade(m));
         m.fields.insert("Transfer-Encoding", "chunked");