Change the queue processing to spawn a new thread for each new event to process (run adapters for).

Previously we had N long-lived threads, where N is set in the configuration setting concurrent_adapters. This is awkward if we want to reload configuration, which is an open feature request.

The new structure is also a lot simpler, which hopefully means it’ll be more reliable.

chore: format code

Because we now use edition 2024, the default formatting by rustfmt is different. Update the formatting, without any other changes, by running cargo fmt.