Only catch signals once

[imurchie]

Proposed changes

Currently we catch every SIGINT and SIGTERM signal coming in and try to stop the http server. Unfortunately, if there are connections, it has to wait for them to be closed on both ends before Appium actually stops.

This is a quick solution, only adding a one-time event listener for the signals. So if someone cares that the connections are closed etc. they can wait, otherwise another signal will terminate the whole thing.

Types of changes

What types of changes does your code introduce to Appium?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• I have read the CONTRIBUTING doc
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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Reviewers: @jlipps

[imurchie]

See #7328

[jmehtaebates]

See #7644

[jlipps]

huzzah, this is much better behavior!