Make `ldk-sample` `Send` + `Sync`

Make `ldk-sample` `Send` + `Sync`

This makes it possible to run ldk-sample in a tokio task, e.g.

```rust
pub async fn main() {
    tokio::spawn(async move {
        start_ldk().await;
    })
        .await
        .expect("LDK panicked")
}
```