Put Redis between Users Service and Firestore

[humphd]

We're going to need to do a lot of reads to the user and feed data in Firestore (see #1828). @chrispinkney and I were discussing some of the limitations that Firestore imposes on reads, most likely because it favours their pricing model.

I want the security and peace-of-mind of having our data in Firestore, but I don't really want to worry about hitting it a million times a day, and incurring read costs.

Another option is for us to transparently cache all Firestore read/write data in our Redis instance, and prefer cached data when we have it. Since all of our interactions with Firebase flow through our microservices, we can have them add a caching layer to the mix.

It looks like this has already been done for us:

https://github.com/ehacke/simple-cached-firestore

There's a nice blog post why he built and and how to use it.

[humphd]

@chrispinkney do you want this, or should I take it?