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/readwritesplit.cc

@@ -36,6 +36,8 @@
 #include "rwsplitsession.hh"
 #include "routeinfo.hh"
 
+using namespace maxscale;
+
 /**
  * The entry points for the read/write query splitting router module.
  *