Improve @timestamp and Date filter parsing speeds

Research/data here: https://github.com/jordansissel/experiments/tree/master/ruby/time-iso8601-parsing

Rough overview:

* Old method: Joda-Time parse to a joda DateTime, construct Time via Time.utc(year, month, ...)
* New method: Joda-Time parse to milliseconds-since-epoch, construct Time via Time.at(...).utc

This will improve performance of anything with Event objects having @timestamp initially as a string, such as things parsed with the JSON codec.

* LogStash::Event.new, when given a @timestamp as sttring, performance up 16x
* JSON codec parsing performance test is up 9.4x
* Date filter parsing speed is up 1.3x