consumer: avoid obscure sync between subscriptionManager and subscriptionConsumer threads
consumer: avoid obscure sync between subscriptionManager and subscriptionConsumer threads

since subscription manager was improved to batch subscriptions (see https://github.com/Shopify/sarama/pull/2109/commits/dadcd808a5f6c6394e91f6a614818cf8eab732de)
it created a deadlock in the case when new subscription are added after a rebalance

DataDog/sarama,Shopify/sarama,DataDog/sarama,Shopify/sarama