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

User of TextParser (== plugin developer) cannot know which parser implementation are used. It is determined by configuration.
So developer MUST set `estimate_current_time = false` before `configuire()` always.