state: reflect key Nomad data within internal store

[jrasell]

This PR moves the policy-storage package to use blocking queries on the scaling policy list endpoint. It also adds processing for job scaling state, which is used to identify jobs that are stopped, and in the future will help provide functionality such as cooldowns.

closes #38

[jrasell]

PR should have commits squashed before merging!

[cgbaker]

good stuff 👍. i left some comments, then some issues which i'm not sure about (e.g., whether we stop tracking deleted policies).

[lgfa29]

Great stuff!

[lgfa29]

On working on #63 I noticed that policies are not being removed when the job is purged (nomad stop --purge). This should probably be handled in the update function.

[lgfa29]

This PR has been updated with the new control loop redesign. The main changes include:

1. Use of blocking queries and channels to communicate state changes from Nomad to the Autoscaler
2. New concepts:
   1. Policy Source: provide channels and methods for a caller to receive updates about the list of policies registered or for a specific policy.
   2. Nomad Policy Source: implementation of a Policy Source to read policies from a Nomad cluster. It uses blocking queries to avoid unnecessary API calls.
   3. Policy Manager: receives updates from a Policy Source and creates/removes Policy Handlers as necessary. It blocks until the list of policies changes, at which point the list of handlers is reconciled.
   4. Policy Handler: monitors a specific policy for updates and submit them back to the agent for evaluation when required. It blocks until the policy being monitored is updated.
3. Update the Target plugin interface to replace Count with Status. Status returns Count in addition to a flag call Ready to indicate if the target is ready to receive scaling actions and Meta tags that will be used in future work.
4. Update the Nomad target plugin to store job status in a local state store and use blocking queries to update it when something changes.

[jrasell]

commits should be squashed when merging.

[cgbaker]

small changes before merge, no need for re-review