refactor(mempool): replace status-poll shutdown with CancellationToken

[zancas]

Parent: #1051

Today

packages/zaino-state/src/chain_index/mempool.rs runs a sync task spawned at :144. The loop body has multiple tokio::time::sleep(100ms / 500ms) calls and a single status.load() == Closing check at :228 near the bottom. close() (:335) stores Closing and calls handle.abort(). Drop (:347) does the same.

Failure modes:

1. Latency: shutdown takes anywhere from one full sleep cycle (100 ms minimum, up to 500 ms in the recoverable-error path) before the task observes Closing.
2. Lost graceful drain: abort() kills the task mid-sleep, so the trailing state.notify(Closing) at :229–230 is skipped. Subscribers never see the explicit closed-state notification.

Proposed change

• Add cancel_token: CancellationToken to Mempool; clone it into the spawned sync task.
• Wrap each tokio::time::sleep(...) call in the sync-task body as:

  tokio::select! {
      _ = tokio::time::sleep(Duration::from_millis(N)) => {},
      _ = token.cancelled() => return,
  }

• The if status.load() == StatusType::Closing check at :228 collapses into the cancellation arm.
• close(): self.cancel_token.cancel(); await handle; — the task exits via the cancelled arm and runs state.notify(Closing) cleanly.
• Drop keeps abort() as a backstop (we may not be in an async context at drop time).

Acceptance criteria

• All tokio::time::sleep(...) calls in the sync-task body are inside a select with cancel_token.cancelled().
• close() no longer calls abort() on the success path; the task exits gracefully and the state.notify(Closing) line runs.
• New unit test asserts close() completes within ~20 ms.
• Existing mempool tests remain green.

Out of scope

• Receiving the token from a parent (zainod) — that's tracking: unify process-wide shutdown via root CancellationToken #1051. Land with Mempool owning its own CancellationToken; child-token wiring follows when zainod owns the root.

[dipanshuhappy]

Hey Happy to work on this. Rust developer, been contributing to rust tooling in ethereum and now exploring Zcash