Improve persisted configuration handling

When a persisted configuration file is read, the values in it are
considered to be more up-to-date than the ones in the main configuration
file. This allows all objects to be persisted in a more complete form
making it easier to change configuration values at runtime.

This change is intended to help make runtime alterations to services
possible.

server/core/maxscale/config.h

@@ -92,6 +92,16 @@ bool config_add_param(CONFIG_CONTEXT* obj, const char* key, const char* value);
  */
 bool config_append_param(CONFIG_CONTEXT* obj, const char* key, const char* value);
 
+/**
+ * @brief Replace an existing parameter
+ *
+ * @param obj Configuration context
+ * @param key Parameter name
+ * @param value Parameter value
+ * @return True on success, false on memory allocation error
+ */
+bool config_replace_param(CONFIG_CONTEXT* obj, const char* key, const char* value);
+
 /**
  * @brief Construct an SSL structure
  *