Switch database connection between reading and writing rollup

[joshuajansen]

First off: Thank you all for the work on Rollup (and all your other brilliant gems), much appreciated. I've been eyeing Rollup forever for our platform's metrics and can't believe I waited this long to implementing it.

As suggested by @dudadornelles here #8, it would be super useful to be able to perform the reads from a replica and only perform the upsert on the primary database. I didn't see an issue for it yet, so consider this my +1 for the idea.

Thanks again!