Faulty move of reference count of shared rmqClient

[wgdzlh]

#888 change reference count of shared rmqClient into sync.Once body, which is buggy. The client will be shutdown once any consumer or producer call Shutdown, but it may be still used by other consumer or producer.
The problem mentioned in #888 will not happen, since the start of client is called in sync.Once:

rocketmq-client-go/consumer/push_consumer.go

Line 161 in 6c77b67

err = pc.defaultConsumer.start()

rocketmq-client-go/consumer/consumer.go

Line 285 in 6c77b67

dc.client.Start()

, so it will only be called once. Multiple start call will not affect the reference count of client.
Simply revert #888 will fix the issue.