When we release too fast, the plugin crashes:
```
thread 'tokio-runtime-worker' panicked at 'called Result::unwrap() on an Err value: SendError(())', plugins/examples/cln-plugin-reentrant.rs:31:27
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
```
This happens with CI under VALGRIND!
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>