Use async event handler and background processor

[tnull]

An async variant of the background processor shipped with 0.0.112 (see lightningdevkit/rust-lightning#1657).

We should switch to this, also requiring us to make the event handler async.

This is kind of straightforward, except for the current implementation of process_pending_events_async holding a RwLockGuard over await boundaries (https://github.com/lightningdevkit/rust-lightning/blob/b79ff71fe7587b03ab0b7fe8b6229eb4bc2d7f08/lightning/src/ln/channelmanager.rs#L5262-L5279), which is not Send and therefore doesn't play with tokio nicely.

We therefore first need to address this upstream, e.g., by introducing a Send lock variant.