Description:

The async disagg step-down path in test/format has two bugs.

1. stepdown_workers_drained() polls all_durable, which only sees timestamps registered via timestamp_transaction(). A worker allocates its commit timestamp before registering it with WT, so it can hold a timestamp at or below step_down_ts that WT doesn't yet know about — the drain can finish early and that commit lands behind stable. Drain on timestamp_minimum_committed() instead (the same signal timestamp_once() uses), and enforce timer-based runs since an exhausted runs.ops budget would stall the drain.
2. disagg_switch_roles() pins stable at step_down_ts without updating g.stable_timestamp, format's cached view of stable, leaving it stale for the entire follower phase. Set g.stable_timestamp = g.stepdown_ts after the set_timestamp call, consistent with timestamp_once().