`is_locked` does not work in a persistent context

I’m working with a persistent storage behind a « in memory » instance of Mongodb. It seems that `Mongod.is_locked` check for lock presence, while `mongod` leave the file (empty of PID) after a successful shutdown..

To reproduce the issue, you simply need to reuse the same directory after a successful shutdown.

I’d suggest that you move the `is_locked` method to something similar to this:

```
@property
def is_locked(self):
lock_file = os.path.join(self.data_folder, "mongod.lock")
if not os.path.exists(lock_file):
return False

return os.path.getsize(lock_file) != 0
```

Search the repository for the `is_locked` method and inspect how the persistent data directory is reused after shutdown. Reproduce the issue by starting and stopping a MongoDB instance, then starting another instance with the same directory. Done means an existing empty `mongod.lock` is treated as unlocked and the reuse case succeeds.

Written by the indexing model from the issue text.