[BACKEND] Use Promise.all instead of for loop in fetch-users

[bcho892]

Is your feature request related to a problem? Please describe.

introduced in #415 was a method to fetch users between a date. while doing #457 we notice there was super slow querying on larger date ranges, and it became apparent that it was because we were doing blocking async calls in a for loop, not taking advantage of concurrency.

We should replace it with a Promise.all

Further reading: https://stackoverflow.com/questions/53798589/await-loop-vs-promise-all

BEFORE MERGING

• [ ] Integration test written for services
• [ ] Schemas annotated if adding new models
• [ ] Code generation run (hint: yarn workspace server tsoa spec-and-routes)
• [ ] Appropriate mocks created where possible
• [ ] PR Reviewed (For non-trivial changes)
• [ ] Changes tested after rebasing on master or merging in master (hint: git fetch origin master:master, then git rebase master or git merge master)
• [ ] All required PR checks passing