Fix execution hangs

[pawelpasterz]

Fixes #649

This PR contains commit 2b92fec which is only small refactor which touches quite big number of files. Please, skip this commit when reviewing.

Changes:

• wrap CommandLine(Main()).execute(*args) with exitProcess(...) which closes jvm (this is workaround for bugsnag bug related to thread pool executor"
• add unit tests

Checklist

• Documented
• Unit tested
• release_notes.md updated

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@dd89178). Click here to learn what that means.
The diff coverage is 59.21%.

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[pawelpasterz]

@bootstraponline @remkop
Thanks for comments, I've added changes you requested, please verify.
Thanks :)

[bootstraponline]

Looks great! Approved. Thanks for updating and adding the test.

[remkop]

The classpath in UtilsTest still has the dummy value classpath...picocli-4.2.0.jar. So that process will die immediately. It won’t hang.

You can get a better classpath from the value of system property "java.class.path".
Sorry if the example was misleading.

[bootstraponline]

Thanks for the follow up clarification! I opened #662 to track this work.

[pawelpasterz]

@remkop Omg, I totally missed that ... shame on me. Thanks for catching this!
Here is fix #663 I also added additional verification logic if process was started.
cc @bootstraponline