* Major refactoring to Lightbus' internals. Resolves a number of long standing issues.
Lightbus now communicates internally using commands sent via queues.
* Python 3.8 now supported
* The `on_error` configuration parameter has been removed and replaced with an
`on_error` argument to `listen()`. [See docs](https://lightbus.org/reference/events/#errors-in-event-listeners)
* Standardised on 'worker' rather than 'server' in naming:
* Renamed `server_started`, `server_started`, and `server_ping`
to `worker_started`, `worker_started`, and `worker_ping`
* Renamed `LightbusServerError` to `LightbusWorkerError`
* Renamed `BusClient.start_server()` to `BusClient.start_worker()`
* Renamed `BusClient.stop_server()` to `BusClient.stop_worker()`
* More cases are now handled in Redis connection retrying (connection refused and Redis in LOADING state).
These would previously result in an unhandled exception.
* Added experimental `--validate` and `--show-casting` arguments to
[lightbus inspect](https://lightbus.org/reference/command-line-use/inspect/)