DRAFT: CIRC-4436 - Allow stacktrace across an eventer asynch

[vasuraman11]

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This branch allows libmtev to stacktrace beyond mtev_run_callback and show what the stacktrace was when eventer_add was called. That allows us to get a greater context if a crash occurs.

[postwait]

I also thing that zipking provides a lot of this already. I think we should close this for a different approach.