Unique CharField with max_length > 255 not allowed by MySQL

```
$ ./manage.py migrate
SystemCheckError: System check identified some issues:

ERRORS:
db_mutex.DBMutex.lock_id: (mysql.E001) MySQL does not allow unique CharFields to have a max_length > 255.
```

This error is fixed by changing max_length to 255 (instead of 256) on the lock_id field in models.py