RADAR-403: Stream resource

Adds the stream resource

1. Uses underlying abstraction of a list-with-autoincrementing-id
2. Supports:
   - standard features: subscribe/unsubscribe/sync/get
   - push: add a message to the end of the stream. This adds a unique incrementing id to the message
   - get(id) - Ability to get the messages from a specific id onwards.
   - subscribe(id) - Ability to subscribe at a given id. All the messages in the list after the id will be resend to the client. This brings support for replaying missed notifications if you remember the last message id you received.
/cc @zendesk/zendesk-radar


### Steps to merge
 - [ ] :+1: of the team

### References
 - Jira link: https://zendesk.atlassian.net/browse/RADAR-403

### Risks
 - Low: None of this code is used, but watch for regression.
Use positive naming: sendEnabled - easier to read - avoids the need to think in double-negatives.