Add some perf tests
Add fast iso8601-string to Ruby Time converter; based on research here: https://github.com/jordansissel/experiments/tree/master/ruby/time-iso8601-parsing
Use much faster "@timestamp" iso8601 string parsing

Also use constants for most strings to avoid creating new string objects
pointlessly.

Running the perf test:
% SPEEDTEST=1 bin/logstash rspec spec/event.rb -e speed

* before patch: event @timestamp parse rate: 19118/sec
* after patch: event @timestamp parse rate: 320718/sec

This was done in response to
https://github.com/octplane/logstash_benchmark in which another user
found something to be slow in logstash and decided to write something
from scratch to replace logstash.

Bugs are not features.
Slowness is a bug. We can improve it, see? :)
Use faster time parsing strategy.

Joda-Time to milliseconds, Time.at(milliseconds / 1000.0).utc

perf tests:

% SPEEDTEST=1 bin/logstash rspec spec/filters/date_performance.rb -e speed
- before patch: date parse rate: 43687.1996505024
- after patch: date parse rate: 57836.899942163094

Research here:
https://github.com/jordansissel/experiments/tree/master/ruby/time-iso8601-parsing
Merge pull request #894 from jordansissel/date-and-json-perf-testing

Improve @timestamp and Date filter parsing speeds