Clean state after subscription failure

[acogoluegnes]

Exception thrown if subscription throws exception:

java.lang.NullPointerException: Cannot read field "client" because "x0" is null
	at com.rabbitmq.stream.impl.ConsumersCoordinator$ClientSubscriptionsManager.access$300(ConsumersCoordinator.java:386)
	at com.rabbitmq.stream.impl.ConsumersCoordinator$SubscriptionTracker.assign(ConsumersCoordinator.java:269)
	at com.rabbitmq.stream.impl.ConsumersCoordinator$ClientSubscriptionsManager.add(ConsumersCoordinator.java:721)
	at com.rabbitmq.stream.impl.ConsumersCoordinator$ManagerPool.add(ConsumersCoordinator.java:340)
	at com.rabbitmq.stream.impl.ConsumersCoordinator$ManagerPool.access$100(ConsumersCoordinator.java:317)
	at com.rabbitmq.stream.impl.ConsumersCoordinator.subscribe(ConsumersCoordinator.java:129)
	at com.rabbitmq.stream.impl.StreamEnvironment.registerConsumer(StreamEnvironment.java:487)
	at com.rabbitmq.stream.impl.StreamConsumer.lambda$new$2(StreamConsumer.java:98)
	at com.rabbitmq.stream.impl.StreamConsumer.<init>(StreamConsumer.java:112)
	at com.rabbitmq.stream.impl.StreamConsumerBuilder.build(StreamConsumerBuilder.java:148)
	at com.rabbitmq.stream.perf.StreamPerfTest.lambda$call$22(StreamPerfTest.java:762)