User ID detection

[MaddyUnderStars]

How can we better detect which IDs Fosscord controls? Currently, we add every ID given to us through gateway or api to a huge list of controlled IDs for each client, however this has a few problems:

• IDs are not unique across instances, which may lead to false redirections
• We may not have an ID in our list before we have to redirect it

The current solution has it's problems, but it's 'fast enough', probably.
A new solution that may or may not be slower is to create some sort of new ID that encodes the old ID and the instance it came from.
It would be best to keep it all numerical, because some instances, applications, whatever may assume they are.