Test failure: LogStash::Util::WrappedAckedQueue with multiple writers ...

Tested using JRuby 9.1.8.0

```
% bin/rspec logstash-core/spec

1) LogStash::Util::WrappedAckedQueue with multiple writers with simulate_work OFF > larger event size < behaves like a well behaved queue writes, reads, closes and reopens
Failure/Error: expect(output_strings).to eq([])

expected: []
got: ["File count after close mismatch expected: 2 got: 4", "page.476", "checkpoint.425", "checkpoint.424"...home/jls/projects/logstash/logstash-core/spec/logstash/acked_
queue_concurrent_stress_spec.rb:118>>"]

(compared using ==)
Shared Example Group: "a well behaved queue" called from ./logstash-core/spec/logstash/acked_queue_concurrent_stress_spec.rb:277
# ./logstash-core/spec/logstash/acked_queue_concurrent_stress_spec.rb:121:in `block in (root)'
# ./vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait.rb:46:in `block in /home/jls/projects/logstash/vendor/bundle/jruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait.
rb'
# ./lib/bootstrap/rspec.rb:13:in `'
```