storage: don't assign JoinHandles to _

Assigning the `JoinHandle` returned by `spawn` to `_` triggers a the
clippy lint `let_underscore_future`. This is a known FP (see
rust-lang/rust-clippy#9932). The easiest way
to avoid it is to not do that assignment.

Author: teskje

src/persist-client/examples/maelstrom/node.rs

@@ -125,7 +125,7 @@ where
             };
 
             let service = Arc::clone(&self.service);
-            let _ = mz_ore::task::spawn(|| "maelstrom::handle".to_string(), async move {
+            mz_ore::task::spawn(|| "maelstrom::handle".to_string(), async move {
                 let service = service.get().await;
                 let () = service.eval(handle, msg.src, body).await;
             });
@@ -164,7 +164,7 @@ where
                 // the AsyncInitOnceWaitable nonsense.
                 let args = self.args.clone();
                 let service_init = Arc::clone(&self.service);
-                let _ = mz_ore::task::spawn(|| "maelstrom::init".to_string(), async move {
+                mz_ore::task::spawn(|| "maelstrom::init".to_string(), async move {
                     let service = match S::init(&args, &handle).await {
                         Ok(x) => x,
                         Err(err) => {

src/persist-client/src/internal/compact.rs

@@ -149,7 +149,7 @@ where
                 let compact_span =
                     debug_span!(parent: None, "compact::apply", shard_id=%machine.shard_id());
                 compact_span.follows_from(&Span::current());
-                let _ = mz_ore::task::spawn(|| "PersistCompactionWorker", async move {
+                mz_ore::task::spawn(|| "PersistCompactionWorker", async move {
                     let res = Self::compact_and_apply(
                         cfg,
                         blob,

src/persist-client/src/read.rs

@@ -933,7 +933,7 @@ where
         //
         // Intentionally create the span outside the task to set the parent.
         let expire_span = debug_span!("drop::expire");
-        let _ = handle.spawn_named(
+        handle.spawn_named(
             || format!("ReadHandle::expire ({})", self.reader_id),
             async move {
                 machine.expire_leased_reader(&reader_id).await;

src/persist-client/src/write.rs

@@ -739,7 +739,7 @@ where
         //
         // Intentionally create the span outside the task to set the parent.
         let expire_span = debug_span!("drop::expire");
-        let _ = handle.spawn_named(
+        handle.spawn_named(
             || format!("WriteHandle::expire ({})", self.writer_id),
             async move {
                 machine.expire_writer(&writer_id).await;

src/storage/src/source/commit.rs

@@ -66,7 +66,7 @@ pub(crate) fn drive_offset_committer<S: OffsetCommitter + Send + Sync + 'static>
 ) -> OffsetCommitHandle {
     let (tx, mut rx): (_, watch::Receiver<HashMap<PartitionId, MzOffset>>) =
         watch::channel(Default::default());
-    let _ = task::spawn(
+    task::spawn(
         || format!("offset commiter({source_id}) {worker_id}/{worker_count}"),
         async move {
             let mut last_offsets: HashMap<PartitionId, MzOffset> = HashMap::new();