separated session from session_impl and made a real pimpl-class of it. Introduced the possibility to destruct the sessions async.

test/test_metadata_extension.cpp

@@ -2,10 +2,13 @@
 #include "libtorrent/hasher.hpp"
 #include <boost/thread.hpp>
 #include <boost/tuple/tuple.hpp>
+#include <boost/filesystem/operations.hpp>
 
 #include "test.hpp"
 #include "setup_transfer.hpp"
 
+using boost::filesystem::remove_all;
+
 void test_transfer(bool clear_files = true, bool disconnect = false)
 {
 	using namespace libtorrent;