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.
Can you expand upon this design decision? I presume this number is guaranteed to be always unique and   monotonically increasing? Is it the message's index in the Raft log? I presume so:

https://github.com/influxdb/influxdb/blob/master/messaging/broker.go#L346