Refactor message and message_headers declarations:

message_headers is now a set of partial class template
specializations instead of a template class alias. This solves
a problem where template functions taking message_headers as a
parameter could not deduce argument types, since std::conditional
obscured the deduced context.

Both classes are refactored to share declarations using an #ifdef,
to eliminate an ugly set of extra declarations needed when building
the documentation.

Copy and move class special members are added.

A new function message::base() is provided which returns the
message_headers portion of a message.

test/http/parser_v1.cpp

@@ -64,7 +64,7 @@ public:
         streambuf rb;
         headers_parser_v1<true, headers> p0;
         parse(ss, rb, p0);
-        request_headers<headers> const& reqh = p0.get();
+        request_headers const& reqh = p0.get();
         BEAST_EXPECT(reqh.method == "GET");
         BEAST_EXPECT(reqh.url == "/");
         BEAST_EXPECT(reqh.version == 11);