Currently, updating a form that has an entity list with many media files takes a long time - our benchmark (EntitiesBenchmarkTest#oneThousandMediaFilesWithEntityList) takes ~15s in a situation where no media files have changes and the entity list is the same. We should get this down to 5 seconds in line with EntitiesBenchmark#oneHundredThousandEntities.

Notes

Experimental changes reversed in #6841 were able to hit the 5 second target here in the entity list/many media file case. The main culprit was another optimization that copies media files from previous versions when performing a form update: this caused every media file to be copied to the download "sandbox" directory from the current version and then back again. This could be avoided if the code was smarter about the fact we're in a "media file update only" scenario. One way to achieve this would be to replace the booleans in ServerFormDetails with an enum (or ADT) to make the download "scenario" we're in explicit: either "new", "new version" or "new media".