Zero-downtime rolling upgrades for Nexus Delta

**Upgrades without downtime**
- When upgrading Delta it would be useful to do it with no downtime.
- If possible, it would mean that users would not be interrupted in the task they were doing when the upgrade happens (depending on the task).

**Describe the solution you'd like**
- When deploying a new version of Delta it should be possible to launch a new Delta instance and have ongoing tasks handed over to the new instance while the old instance is being shut down.
- A zero-downtime upgrade should be possible both in a cluster of Delta instances as well as in the single Delta instance use case.

**Possibly related to the following issues**
- https://github.com/BlueBrain/nexus/issues/729

The issue names no files, tests, or entry points. Start by reviewing related issue #729 and the existing deployment, clustering, and task-handling behavior. Done would require a defined zero-downtime upgrade path for both clustered and single-instance Delta deployments.

Written by the indexing model from the issue text.