Rewrite the chain listener as a single loop. (#3745)

## Motivation

For #365 the chain
listener will need to react to new block notifications on a publisher
chain by processing the inboxes of all subscriber chains. That is
difficult to implement with the current approach: we run a notification
processing loop in a separate task for each chain.

## Proposal

Rewrite the chain listener so that there is a single top-level loop
processing all events on all chains. That will make it easier to update
chains based on events from _other_ chains.

`process_new_events` already does this, to update all non-admin chains
for new epochs.

## Test Plan

CI

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

- Preparation for #365.
- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

Author: afck

Cargo.lock

@@ -138,10 +138,10 @@ mod implementation {
     }
 
     /// The type of a future awaiting another task.
-    pub type NonblockingFuture<R> = oneshot::Receiver<R>;
+    pub type NonBlockingFuture<R> = oneshot::Receiver<R>;
 
     /// Spawns a new task on the current thread.
-    pub fn spawn<F: Future + 'static>(future: F) -> NonblockingFuture<F::Output> {
+    pub fn spawn<F: Future + 'static>(future: F) -> NonBlockingFuture<F::Output> {
         let (send, recv) = oneshot::channel();
         wasm_bindgen_futures::spawn_local(async {
             let _ = send.send(future.await);

linera-base/src/task.rs

@@ -16,7 +16,6 @@ test = ["linera-views/test", "linera-execution/test"]
 benchmark = [
     "linera-base/test",
     "dep:linera-sdk",
-    "dep:tokio-util",
     "dep:crossbeam-channel",
     "dep:num-format",
     "dep:reqwest",
@@ -80,7 +79,7 @@ thiserror.workspace = true
 thiserror-context.workspace = true
 tokio.workspace = true
 tokio-stream.workspace = true
-tokio-util = { workspace = true, optional = true }
+tokio-util.workspace = true
 tracing.workspace = true
 trait-variant.workspace = true