Get rid of pickle.

```python
self.db = PickleShareDB(os.path.join(self.profile_dir.location, 'db'))
```

is like 

```python
self.db = exec((Path(self.profile_dir.location) / 'backdoor.py').read_text())
```

, but worse: at least we can audit contents of `backdoor.py` with a text editor.
