timestamp performance improvement

- The `Timestamp#to_json` method was very very slow, this fix improves json serialization performance. 
- Also added a cleanup and performance improvement to `Event#to_s`
Thank you @wibaa I am resurrecting the mri branch, I'll scout the issues
for other fixes.

In lib/logstash/timestamp.rb:

>      end
>      alias_method :inspect, :to_json
>
>      def to_iso8601
> -      @time.iso8601(ISO8601_PRECISION)
> +      @iso8601 ||= @time.iso8601(ISO8601_PRECISION)

What about the date filter? that one updates the timestamp right? or am i
understanding it wrong?

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