modify Job.Scale RPC to return an error if there is an active deployment

[cgbaker]

Job.Scale will return a 400-coded error if the job has an active deployment. This is a short-term solution to the problem where the autoscaler might attempt to scale the job while it is being updated, which would result in the previous deployment being cancelled and a new deployment being created.

This PR does two things:

• block the Job.Scale operations
• make a best-effort attempt to register a scaling event in the case that the scaling action was blocked

resolves #8143

[tgross]

LGTM! 👍

[tgross]

The nomad/volumewatcher TestVolumeWatch_StartStop that's showing up in https://app.circleci.com/pipelines/github/hashicorp/nomad/9948/workflows/bb354ce0-6e80-42b0-adcc-9bbcdfc218ff/jobs/76232/steps isn't a known-flaky test (or at least I've never seen it flake). But I can't see any way this code would impact it, so I'm going to kick off the build again and then open a new issue to look into that as part of the CSI work I'll be digging into post 0.12.0-beta

[tgross]

Flaky test fixed in #8189

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.