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

[vasuraman11]

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]

This increases the eventer_t by over a kilobyte.

This should be implemented as an optional module that leverages the existing eventer_context_t as zipkin does.

[postwait]

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