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.

Look at the database handling code, likely using peewee, to understand how concurrent access is managed. Check for file locks or database locks in the codebase. The goal is to implement a locking mechanism or queue so multiple instances can run without corrupting messages or causing database errors.

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