Fix race condition waiting for rounds. (#3099)

## Motivation

`test_end_to_end_listen_for_new_rounds::storage_test_service_grpc` is
flaky for me locally and sometimes hangs, even though it seems fine on
CI.

The logs suggest that the two clients are both waiting for round
notifications forever, even though one of them should be the leader in
the current round.

## Proposal

Adding an explicit `prepare_chain` fixes the issue.

It's not quite clear why this is necessary, so I created
#3100.

## Test Plan

Running the test in a loop locally failed quite often. With this change
it passed 50 times in a row.

## Release Plan

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

## Links

- Issue to revisit this:
#3100
- Bug was introduced/revealed in:
#2961
- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

linera-client/src/client_context.rs

@@ -435,6 +435,7 @@ where
 
         loop {
             // Try applying f. Return if committed.
+            client.prepare_chain().await?;
             let result = f(client).await;
             self.update_and_save_wallet(client).await?;
             let timeout = match result? {