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`
Correct. The `Timestamp` is practically immutable so there is no problem in memoizing the `to_is08601` call. 

I say *practically* because there is one method which mutates its internal `@time` state, it is `Timestamp#utc` but a Timestamp is always initialized as UTC so calling `Timestamp#utc` will not really  change it. This method was added for compatibility with plugins which were originally calling the `utc` method.