Feature Request: make it work well when several instances are run concurrently

```bash
for i in $(seq 1 10); do matrix-commander --message $i & ; done
```

Running the above yields:

- One successful message to the channel
- Two `peewee.OperationalError: database is locked` errors
- Six corrupt messages in the channel `Unable to decrypt: The secure channel with the sender was corrupted`
- One `ERROR: matrix-commander: Message send failed. Sorry.` with no traceback

It would be excellent if multiple instances of this program could run concurrently, or at the very least wait politely for a while for any siblings to finish.