MXS-2200: Add runtime handling for all global parameters

All global parameters are now handled by the runtime configuration
modification code. The parameters that are trivial to update can now be
updated at runtime. All other global parameters cause a new error message
to be returned stating that the parameter in question cannot be modified
at runtime.

Also updated the list of modifiable parameters in MaxCtrl. This list
should not be stored in MaxCtrl and should be created by MaxScale at
runtime.

server/core/internal/config.hh

@@ -227,3 +227,12 @@ void dump_param_list(int file,
                      const std::unordered_set<std::string>& ignored,
                      const MXS_MODULE_PARAM* common_params,
                      const MXS_MODULE_PARAM* module_params);
+
+/**
+ * Check whether a parameter can be modified at runtime
+ *
+ * @param name Name of the parameter
+ *
+ * @return True if the parameter can be modified at runtime
+ */
+bool config_can_modify_at_runtime(const char* name);