utils/ssh: Fix atexit.register() SshConnection leak #679
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SshConnection registers an atexit handler so the connection is closed upon exiting the process if it has not been done before. However, the handler keeps a reference on the connection, which means it will stay alive. If lots of short-lived connections are created (which can happen when using e.g. ThreadPoolExecutor), they will simply stay around and leak.
Fix that by using a weak reference (WeakMethod) to register in the atexit handler, with a callback to unregister it when the object is deallocated.
Fixes #676