Add config_param "time_default_current" not to set current time into parse result

 * By setting current time, caller cannot know "time" is from whether parse result or current time
   * it brings troubles especially in case for filter plugins like fluent-plugin-parser
 * naming problem:
   * 'time_' prefix seems important for me

No. BEFORE.
It is because ValuesParser raises error on configure when time_key is defined and it is not included in keys.