Disregard stale DynamicMapLayer updates

[davetapley]

Alleviates #749 by not updating layer with image for a set of params which have changed while request was in flight.

This isn't the most elegant solution; I'm very open to feedback 😁

[jwasilgeo]

Hi @davetapley, thank you for offering this change, we appreciate it! 👍

As a holistic, longer-term update, I'd like to see esri-leaflet's requests mechanisms be updated to support network requests that can be aborted/canceled, as folks have mentioned in #749. It is on our radar to evaluate and consider that more closely, as it would be a bit of an update that would likely lead to a major version release.

I think for now we'll leave your proposed change parked here while we figure out next steps.

[AStasyK]

Hello @jwasilgeo, We're having the same issue in our application. I hope the solution is progressing. It will be very important!