Un-reference intervals and timeouts

[pfumagalli]

We observed on production that sometimes (every few or so months) when a Lambda function gets terminated because of a timeout, locks are still kept active somehow (yet unclear why).

We assume that the node process might be kept alive by the setInterval used to refresh and extend the lock, so we are now unref()-ing it to see if it alleviates the issue.

Also, we're adding a test forcedly exiting the node process to verify that the lock is automatically released.