Use current thread runtime for shutdown

Previously, we'd use the node's spawned multithread runtime to await the
event handler finishing up. However, this would require a
`block_on`/`block_in_place` dance, the former of which could still end
up locking up if we happen to be out of blocking threads on shutdown.

Here, we instead avoid touching the runtime and use a new current thread
runtime to wait for event handler shutdown.

Author: tnull

src/lib.rs

@@ -643,9 +643,10 @@ impl Node {
 	///
 	/// After this returns most API methods will return [`Error::NotRunning`].
 	pub fn stop(&self) -> Result<(), Error> {
-		let runtime = self.runtime.write().unwrap().take().ok_or(Error::NotRunning)?;
+		let mut runtime_lock = self.runtime.write().unwrap();
+		let _runtime = runtime_lock.take().ok_or(Error::NotRunning)?;
 		#[cfg(tokio_unstable)]
-		let metrics_runtime = Arc::clone(&runtime);
+		let metrics_runtime = Arc::clone(&_runtime);
 
 		log_info!(self.logger, "Shutting down LDK Node with node ID {}...", self.node_id());
 
@@ -675,8 +676,10 @@ impl Node {
 		// FIXME: For now, we wait up to 100 secs (BDK_WALLET_SYNC_TIMEOUT_SECS + 10) to allow
 		// event handling to exit gracefully even if it was blocked on the BDK wallet syncing. We
 		// should drop this considerably post upgrading to BDK 1.0.
+		let stop_runtime =
+			tokio::runtime::Builder::new_current_thread().enable_time().build().unwrap();
 		let timeout_res = tokio::task::block_in_place(move || {
-			runtime.block_on(async {
+			stop_runtime.block_on(async {
 				tokio::time::timeout(
 					Duration::from_secs(100),
 					event_handling_stopped_receiver.changed(),