WIP: Refactor database methods into server

## Overview

This pull request consolidates the `Database` API into `Server`. This simplifies a lot of the code flow -- especially the messaging.

I also consolidated `ClusterAdmin` and `DBUser` together. The matchers have been removed for now and I'll add per-DB permissions back in when we do authentication on the `Handler`.
I agree that `POST` is non-RESTful since we're not creating IDs. But I feel like `POST`ing-to-create is a common idiom and most people don't understand REST anyway. :)