Add more session command related functions to Backend

The class now allows simpler construction of session commands by
overloading the add_session_command with a version that accepts a const
reference to a shared pointer. This removes the need to copy the
references to the source buffer by calling gwbuf_clone.

Exposed the first session command as a const reference to allow
interaction with it. Currently, it is planned to be used to get the
session command position of each backend.

include/maxscale/backend.hh

@@ -73,6 +73,7 @@ public:
      *                 the session command is completed
      */
     void add_session_command(GWBUF* buffer, uint64_t sequence);
+    void add_session_command(const SSessionCommand& sescmd);
 
     /**
      * @brief Mark the current session command as successfully executed
@@ -90,6 +91,20 @@ public:
      */
     size_t session_command_count() const;
 
+    /**
+     * @brief Get the first session command
+     *
+     * Returns the first session command in the list of session commands
+     * to be executed.
+     *
+     * This should only be called when at least one session command has been
+     * added to the backend. If no session commands have been added, behavior
+     * is undefined.
+     *
+     * @return The first session command
+     */
+    const SSessionCommand& next_session_command() const;
+
     /**
      * @brief Get pointer to server reference
      *