Add a 10 second max timeout to flush requests

[benjackwhite]

When shutting down, users are advised to call .flush to ensure all queued events are sent. Without a timeout this can block the shutdown, causing other issues

[neilkakkar]

Neil to check if timeouts on requests are sufficient to not block thread joins

[0xRaduan]

How long does flush take in general? I just read about using it here, and I have a bunch of short-lived tasks in Celery. However, what I've noticed, is that after putting flush at the end of my celery task, it's blocked(for >10 minutes), and never actually finishes / nor sends then request.

For now I am using sync client, but I want to use async one, because otherwise I have celery tasks, which can produce a lot of events, and I don't want to use sync calls for that.

Please advise.