Add flat_stream to experimental:

This adds a new stream wrapper class template designed to address
a performance shortcoming of boost::asio::ssl::stream.

test/example/common/Jamfile

@@ -9,7 +9,6 @@
 
 local SOURCES =
     detect_ssl.cpp
-    flat_stream.cpp
     root_certificates.cpp
     server_certificate.cpp
     session_alloc.cpp