Remove custom parser

One incompatibility change is included.

When input stream includes invalid time value (e.g. `{"time":[]}`), old implementation pass through `time` value.
New implementation handles it as error because Fluent::TextParser raise an exception.

`estimate_current_event` must be set before `configure()`.
See comment here https://github.com/fluent/fluentd/blob/master/lib/fluent/parser.rb#L656