Add node management

## Overview

This pull request adds node listing, creation & deletion to the `Server` and the HTTP endpoints. These nodes represent the data nodes in the cluster. This will be used for shard assignment in a future PR.
I was using it because it was an easy way to get a consistent id but now that I'm thinking about it a monotonically increasing id would look nicer. I changed it to use Bolt's `NextSequence()` here: 957ea91