Do proper clean-up of tmp directories

[cmaglie]

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• [X] The PR has no duplicates (please search among the Pull Requests before creating one)
• [X] The PR follows our contributing guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)

What kind of change does this PR introduce? Do proper cleanup of temporary folders.

What is the current behavior? Temp directories are not removed after receiving the shutdown message, see https://github.com/arduino/arduino-language-server/issues/145 for details

What is the new behavior? The shutdown message is now properly handled and the temp directories are removed.

Other information: Partially resolve #145