Why is Logstash so slow to start?

Here are some profilling I did on master, where a simple pipeline takes 9 seconds to start executing on my macbook:

```
% bin/logstash -e "input { generator { count => 1 } } output { stdout { codec => dots} }"
bash script Thu Jun 16 11:07:41 WEST 2016
started setting up bundler at 2016-06-16 11:07:43 +0100
require bundler took: 0.359
patching bundler took: 0.56
bundler reset took 0.017
bundler setup took 3.763
done setting bundler 2016-06-16 11:07:48 +0100
runner#run 2016-06-16 11:07:50 +0100
agent#execute 2016-06-16 11:07:50 +0100
Pipeline main started
.Pipeline main has been shutdown
```

This means that, of the 9 seconds it takes to execute the pipeline, we spend:

~2 seconds going from BASH to JRuby
~5 seconds executing `LogStash::Bundler.setup!({:without => [:build, :development]})`: https://github.com/elastic/logstash/blob/master/lib/bootstrap/environment.rb#L64
~2 seconds processing cli args, creating the agent, compiling the pipeline and other setup tasks