Have sync_wallets use a multi-threaded runtime

As part of the `OuputSweeper` we broadcast in the `Confirm`
implementations, i.e., this would be called during transaction syncing.
Since our broadcasting happens async but via the intermediate blocking
`BroadcasterInterface` we have to transition from `sync_wallets`
(blocking) -> LDK/BDK wallet sync (async) -> Confirm (blocking) ->
BroadcasterInterface (blocking) -> bdk::Wallet::broadcast (async).

However, as the `block_in_place`/`block_on` combo is only usable for
multi-threaded runtimes, this new codepath now requires the use of a
multi-threaded runtime in `sync_wallets` (note this is the case even
though we use a stored, *different* multi-threaded runtime for the
actual broadcasting. **sigh**).

src/lib.rs

@@ -988,7 +988,7 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 		];
 
 		tokio::task::block_in_place(move || {
-			tokio::runtime::Builder::new_current_thread().enable_all().build().unwrap().block_on(
+			tokio::runtime::Builder::new_multi_thread().enable_all().build().unwrap().block_on(
 				async move {
 					let now = Instant::now();
 					match wallet.sync().await {