Issue Summary

When a migration of repositories to Mesh is stopped via the UI or cancelled via the REST endpoint, the migration job can stay in the cancelling state for a long time. API documentation says it should not take more than a few seconds to cancel.

Steps to Reproduce

UI

1. Navigate to Admin > Bitbucket Mesh > Migrate repositories
2. Select a large repository and begin migration by clicking "Migrate"
3. Click "Stop" on the migration, and "Stop" in the dialogue

API

1. Perform steps 1 and 2 from above
2. Invoke the Cancel Mesh migration job API endpoint for the running migration 

Expected Results

UI

The migration progress bar has text that informs users that cancellation may take a long time. e.g. "The migration is being cancelled. This will complete when the in-progress repository has finished migrating"

API

Cancel Mesh migration job API endpoint that describes the cancelling behaviour accurately. e.g. "Cancellation may take significant time. It completes when the in-progress repository has finished migrating"

Actual Results

The migration completes the in-progress repository before cancelling the job.

The UI says "Stopping" for a long time with no context. Users may think it's stuck.

The API documentation says the cancellation should take no more than "a few seconds".

Workaround

No workaround needed. Wait for the cancellation to complete.