databendlabs / openraft / 27361362374
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drmingdrmer 
feat: removed leader transfers leadership before stepping down

# Summary

A Leader removed from a committed membership config now steps down
gently: after a configurable delay it transfers leadership to the most
up-to-date voter, then reverts itself to a learner as a guaranteed
fallback. The automation runs as a task watching the metrics, outside
the consensus kernel, replacing the tick-driven step down inside
`RaftCore`.

# Details

- Add `Config::removed_leader_step_down: StepDownPolicy`: `After(ms)`
  (default `After(150)`) is the delay counted from the commit of the
  membership config that removes the Leader; `Never` disables the
  automation so the application reverts the Leader manually with
  `Trigger::refresh_server_state()`. In CLI the value is either a
  "never" literal (`never`, `no`, `none`, `off`, `false`,
  case-insensitive) or the number of milliseconds; a config file
  without this field keeps the default via a serde default.

- `StepDownWatcher`, spawned in `Raft::new()`, waits on the server
  metrics channel, which changes only on server state transitions,
  including the commit of a membership config. When this node is a
  Leader not contained in the committed membership config, it sends
  `TriggerTransferLeader` targeting the voter with the greatest
  matching log id, then after another `heartbeat_interval` sends
  `RefreshServerState`. Both commands re-validate the state inside
  `RaftCore`, so acting on stale metrics is a no-op. It holds a weak
  API sender and quits when `RaftCore` terminates.

- Remove the tick-driven step down from `RaftCore`, which polled the
  condition on every tick and reverted the removed Leader without
  transferring leadership, leaving the new cluster to elect a Leader
  only by election timeout.
Source Files on build 27361362374
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