Globally limit number of parallel network requests

[bdmendes]

          Since the number of courses here is unbounded, we should find a way to only have 2 or 3 requests active at a time and then go through the list as futures are completed. Having too many active pending requests may slow down the process and/or cause some to fail

Originally posted by @limwa in https://github.com/NIAEFEUP/uni/pull/1116#discussion_r1482207167