Add gevent pool support

[sidmitra]

If we're using gevent as the worker pool, is there some identifier to track the green thread? Can we export that alongside?
This will allow us to figure out if the —concurrency=15 really is working or some of the eventloops are hung.

[HassankSalim]

Will explore this, but from what i have seen till now worker related info (like worker-id) are not present in task events, we might have to separately use worker events to figure this out.

[sidmitra]

We aren't using gevent anymore. But we might still add support for this.