_handle_events called _save_cursors inline whenever a batch moved the
cursor. It ends in atomic_json_write (mkstemp + fsync + os.replace), and
on the WebSocket transport _handle_events runs once per inbound EVENT
frame, so every message paid an fsync on the gateway's event loop,
stalling every other adapter and in-flight turn for its duration.
Split the snapshot from the write: the payload is still built on the
loop (_channel_state is loop-owned), and the write goes through
asyncio.to_thread. The snapshot is taken under an asyncio.Lock so a
slower, older write can never land after a newer one and regress the
durable cursor. connect() keeps the synchronous _save_cursors.
(cherry picked from commit 299929741429262f497b86a67940ffb37faa1694)
b331e5d282