Refactor updateUnknownsAndDuplicates to only check certain wells, making it more efficient

Recapping this issue: I wrote the following comment in `patchWell()`—
https://github.com/biocore/LabControl/blob/10ef1ef11f677957f23dc9765235e5642a81e1e1/labcontrol/gui/static/js/plateViewer.js#L458-L472

@AmandaBirmingham responded to it [here](https://github.com/biocore/LabControl/pull/585#discussion_r322494558)—

>Probably. `updateUnknownsAndDuplicates` is completely agnostic to whatever change triggered it--but that means it has to be brute-force and re-check everything. I think it is the source of a lot of the performance issues in the platemapper, and probably needs to be rethought from the ground up. Could you add an issue to the repo for this, and then remove this comment?