go-nsq: connection pool is not goroutine safe

Reader.nsqConnections is a map holding all connections to nsqd.

In lookupdLoop goroutine, it will check and fix all the non-live connections involving inserting  into  *nsqConnections*. 

In stopFinishLoop()，it will delete the closed connection in *nsqConnections*. 

So, there exists race condition. *nsqConnections* map structure can be damaged.