Cache valid API keys

[Carifio24]

This PR adds a map of API keys that the server has found to be valid and stores them locally. This gives us the benefit of improved performance (no need for a database query) while still allowing the database to be persistent storage. If the server is reset/a new server is spun up, it can just build up the cache as it gets requests.

A few notes:

• Our total number of unique API keys is small, so this shouldn't use much space in memory.
• This doesn't do anything with invalid API keys. I guess there's no reason that we couldn't, but I don't really expect this to be a concern - we're more likely to get requests that don't have a key. Also, there's always the (very) slim chance that we add a new API key that clashes with something that has already been marked bad.
• This does mean that updates to an already-used API key (e.g. permissions) would require a server restart. We basically never change these, but it's worth noting in case this ever happens.