Move RWBackend into a separate file

Moved the RWBackend class implementation into its own file. Made some of
the command type functions a part of the <maxscale/protocol/mysql.h>
header to make it reusable.

server/modules/routing/readwritesplit/rwsplit_session_cmd.cc

@@ -22,6 +22,8 @@
 
 #include <maxscale/router.h>
 
+using namespace maxscale;
+
 /**
  * Functions for session command handling
  */