Implement firestore caching with getDocuments on the server

[AndrewLester]

At the moment, caching is simply disabled on the server because localStorage doesn't exist:
https://github.com/apc-gmhs/mustang-mug/blob/4a87e0bbfd8a70fa56c58ef0fe19eb2c233ea007/src/lib/query.js#L58-L62
Instead, we could just set up an object on the server for this or use something like redis. The object has to persist through requests, which I'm not sure how to set up exactly. Just stick whatever we do in an else here, and in the function below it for reading the value.