MM-53747: Do not start if job is in-progress as well. (#24115)

We missed this out last time. It's possible in an HA
scenario for a second pod to start later while the other
job is in-progress. In that case, it would schedule
two jobs.

https://mattermost.atlassian.net/browse/MM-53747

```release-note
NONE
```

server/channels/store/store.go

@@ -713,6 +713,10 @@ type ReactionStore interface {
 
 type JobStore interface {
 	Save(job *model.Job) (*model.Job, error)
+	// SaveOnce will only insert the job with the same category once.
+	// If this method is called concurrently with another job of the same type,
+	// then nil, nil is returned.
+	SaveOnce(job *model.Job) (*model.Job, error)
 	UpdateOptimistically(job *model.Job, currentStatus string) (bool, error)
 	UpdateStatus(id string, status string) (*model.Job, error)
 	UpdateStatusOptimistically(id string, currentStatus string, newStatus string) (bool, error)