When targeting a specific printer, the scheduler's power-on-wait loop
created new MQTT clients on each attempt. Each new client re-tried the
request topic subscription, which some brokers (e.g. A1) reject by
disconnecting. This caused a 170s thrash loop leaving the connection
fragile, so the eventual print command silently failed to reach the
printer.
Cache request topic support per serial number at the class level so
new client instances inherit the knowledge and skip the subscription.
maziggy
750accb17d