Fix race condition waiting for rounds.

[afck]

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: Debug test_end_to_end_listen_for_new_rounds::storage_test_service_grpc notification issue. #3100
• Bug was introduced/revealed in: Remove redundant prepare_chain calls. #2961
• reviewer checklist