When two forms have the same Entity List attached, it is downloaded twice

[lognaturel]

This is related to a Central implementation detail that may not be relevant to other server implementations.

Central only gives App Users access to Entity Lists through the forms they are attached to. Those Entity Lists are served as form attachments with URLs that include the form id. This means that if you have 3 forms that all attach the same Entity List, that Entity List will be served through 3 different URLs. This makes the OKHTTP caching that we have in place unhelpful for reducing what gets downloaded.

Here are some approaches to this which we've discussed:

• within one form download session, keep track of which Entity Lists have been downloaded and don't download each more than once
• keep track of the ETag of an Entity List the last time it was downloaded indexed by the Entity List name and only download it again if the ETag has changed

Related: #6030