time: Add EventTime#to_time for faster Time creation

[repeatedly]

Signed-off-by: Masahiro Nakagawa repeatedly@gmail.com

Which issue(s) this PR fixes:
None

What this PR does / why we need it:
Fluent::EventTime can be converted into Time via Time.at but
this approach is slower. Output plugins sometimes require Time
for client libraries so adding faster method.

Here is benchmark code:

require 'benchmark/ips'
require 'fluent/time'

ot = Time.now
et = Fluent::EventTime.from_time(ot)

Benchmark.ips do |x|
  x.report "to_time" do
    et.to_time
  end

  x.report "Time.at" do
    Time.at(et)
  end
end

Warming up --------------------------------------
             to_time   171.865k i/100ms
             Time.at    92.733k i/100ms
Calculating -------------------------------------
             to_time      2.861M (± 5.1%) i/s -     14.265M in   5.002172s
             Time.at      1.184M (± 4.1%) i/s -      5.935M in   5.022106s

Docs Changes:
No need for now.

Release Note:
Same as title

[ganmacs]

the change looks good but 2.4 or before tests failed.
https://travis-ci.org/fluent/fluentd/jobs/548400039

[repeatedly]

That's weird. This test passed with ruby 2.4.6 on my Mac...

[repeatedly]

Test now passed on older environment