Cancelling a deferred from `create_deferred` failed it at once and left
the tokio task running to completion, its result thrown away. That is
the right behaviour for a cache load, which other callers may be
waiting on, but wrong for work only the cancelled caller wanted: a
`/sync` request's per-room loop, say, which clients abandon constantly
and which holds a pooled connection while it runs.
`create_deferred_with` takes an `OnCancel`. With `Abort`, the deferred's
canceller aborts the task, so its future is dropped where it stands and
whatever it holds is released by `Drop`. The default stays as it was.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0194eNQP5weBtVMg8NKtc6VU
