Make the Redis engine's PUB/SUB subscriptions concurrency-safe

Make the Redis engine's PUB/SUB subscriptions concurrency-safe: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). The Redis engine drives all of its channel fan-out through a single Redis PUB/SUB connection. Under load this falls apart: many client connections call…