Implement last ok in events

[amdprophet]

Add a last_ok field to events that will get updated with a timestamp every time an event with an exit status of 0 is received by a backend.

Sensu 1.x implementation: https://github.com/sensu/sensu/blob/master/lib/sensu/server/process.rb#L584-L586

[zbintliff]

I imagine the best place for this is: https://github.com/sensu/sensu-go/blob/master/backend/eventd/flapping.go#L27

If so, I may try to take a stab later today

[palourde]

Hi @zbintliff,

That's definitively the right place! If you have any question or need assistance with this, don't hesitate to join us in our Community Slack #sensu2 channel.

Thanks, this could really be an awesome functionality to add!