Move Backend and SessionCommand classes to the core

Using the same implementation of the Backend and SessionCommand classes in
both schemarouter and readwritesplit will prevent duplication of code.

This commit only splits the generic parts of the class to a Backend class
which the schemarouter then extends. The session commands for both routers
are similar so they require no special handling.

server/modules/routing/schemarouter/schemarouterinstance.hh

@@ -22,9 +22,8 @@
 
 #include "schemaroutersession.hh"
 
-using std::string;
-using std::set;
-using namespace schemarouter;
+namespace schemarouter
+{
 
 class SchemaRouterSession;
 
@@ -54,3 +53,5 @@ private:
     SPINLOCK              m_lock;          /*< Lock for the instance data */
     Stats                 m_stats;         /*< Statistics for this router */
 };
+
+}