Fix file descriptor leak in logger reinitialisation logic and print stack trace in severe logs

[nikita-tkachenko-datadog]

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What does this PR do?

Fixes a file descriptor leak in logger reinitialisation logic.
Whenever there is problem with the logger, it is reinitialized.
Reinitialisation entails creating a new logger instance.
The problem is that the old instance is not being closed.
The logger uses a SocketHandler, which opens a socket.
This handler needs to be closed, otherwise file descriptor that corresponds to the opened socket "leaks".
If the logger is reinitialised many times the process runs out of available file descriptors.

Description of the Change

Whenever the logger is reinitialised, previously existing instance (if any) is properly closed.

Alternate Designs

Possible Drawbacks

Verification Process

Tested manually using a dockerized Jenkins instance.

Additional Notes

DatadogUtilities#severe was changed to log exception stack trace: the stack trace is the most valuable and useful piece of info, and it is rather troublesome to ask customers to tweak their log levels and reproduce their problem all over again because the exception was not logged the first time.

Release Notes

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• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
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[sarah-witt]

Nice!