We cannot always reset `readyToSubscribeOnStart` to `false` in the `stop()`
since we may be connected to `ClientManager` and have `cleanStart` option,
so we `unsubscribe()` in the stop and then when we start there is no way to subscribe back
* Reset `readyToSubscribeOnStart` in the `AbstractMqttMessageDrivenChannelAdapter.doStop()`
only if we don't unsubscribe for a `cleanSession` reason.
* Also reset it in the `connectionLost`, so next `connectComplete` will take care about subscription
or `readyToSubscribeOnStart` state change
* Use `isActive()` instead of `isRunning()` in the `connectComplete()` since there is a race
condition when connection is established but `doStart()` has not returned yet, but already passed
`if (this.readyToSubscribeOnStart)` line
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