Improve startup time by using jruby-bin instead of jar.

The major change in vendoring here is that we don't use jruby-complete
anymore. We now use bin/jruby as the entry point.

This improves the startup time of Logstash on my laptop by approximately
1.5 seconds (~25% speedup of startup time)

Testing `bin/logstash version` speed; 5 runs each:
* Before patch: 7.27 7.55 7.26 7.42 7.47
* After patch: 5.54 5.97 5.67 5.58 5.54

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This also updates untar() to respect file permissions and skip copying
if the file size and permissions are correct. This should help speed
up repeated `rake` runs as well as improving correctness.