add support for go-cache backend

[jbain]

This adds support for a simple, ephemeral, in-memory backend. It's a pretty quick proof of concept. I don't have strong opinions for the best way to configure/signal which db backend to use.

Let me know your thoughts!

To run:

 SPIRIT_CONNECTION_URI="asdf" SPIRIT_DB_DRIVER=ephemeral make run

[lukewhrit]

To keep in line with the other database drivers, I think we can just do SPIRIT_CONNECTION_URI="cache://" or something along those lines.

I do wonder if there is a better library to use since go-cache seems to be unmaintained and if it should be actually be persisted to disk or not? An ephemeral database could be very useful for testing Spacebin in a more robust fashion compared to the current setup.

[jbain]

SPIRIT_CONNECTION_URI="cache://" works for me, I'll make the change.

re persistence: For my use case I am looking for something that runs entirely within memory with no saved state. It makes testing easy, and also allows for stateless deployments where there is no expectation of persistence.

re: github.com/patrickmn/go-cache it's a pretty basic wrapper around go maps, I've used it in production applications several years, though it has gone fairly stagnant. There may be more modern and feature rich implementations, but this one works well for what it does. If you feel strongly, I can look into other implementations.

[lukewhrit]

Sounds good. Frankly, as long as go-cache is stable, I have no problem with it.