feat: notify about potential serialization failures when sending DagRun, don't serialize unnecessary params, guard listener for exceptions

[mobuchowski]

We've moved to a model where DagRun level events utilize ProcessPoolExecutor to send data, to not do this on the direct scheduler process. This PR adds more guards to that model:

• do not send/pickle whole DagRun model to process, since serializing only selected attributes is cheaper and less error prone
• add notification on ProcessPoolExecutor complete - this allows listener to log potential serialization failures.
• guard whole listener with BaseException checks - we don't want errors to bubble to scheduler.