refactor(tasks): separate coordinator and middleware

[GeorgeMac]

This is a WIP

This separates the current task coordinators reponsibilities into both a coordinator type and a task service middleware.

The task service middleware handles delegation to a persistent task service implementation and then notifies a middlware.Coordinator implementation that the action has taken place.
It is the job of the new coordinator.Coordinator to delegate the appropriate actions to a backend.Scheduler implementation.

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)
• Documentation updated or issue created (provide link to issue/pr)
• Signed CLA (if not already signed)

[lyondhill]

LGTM :) mostly just comments, with one little feature request. Approving :)