Add an index on process_manager_executables

| Q                | A
| ---------------- | -----
| Bug report?      | no
| Feature request? | yes
| BC Break report? | no
| RFC?             | no
| Branch?          | master

```sql
alter table `process_manager_executables` add index `idx_active_cron` (`active`,`cron`)
```

As suggested by Azure's MySQL performance recommendations.