MXS-1502: Add have_session_commands helper function

The function conveys the meaning of the call better than using
session_command_count to check whether there are session commands to be
executed.

include/maxscale/backend.hh

@@ -87,12 +87,22 @@ public:
     uint64_t complete_session_command();
 
     /**
-     * @brief Check if backend has session commands
+     * @brief Get number of session commands
      *
      * @return Number of session commands
      */
     size_t session_command_count() const;
 
+    /**
+     * @brief Check if there are session commands waiting to be executed
+     *
+     * @return True if there are session commands waiting to be executed
+     */
+    inline bool have_session_commands() const
+    {
+        return !m_session_commands.empty();
+    }
+
     /**
      * @brief Get the first session command
      *