nsqlookupd: remove registration when last producer unregisters ephemeral topic
nsqlookupd: remove registration when last producer unregisters ephemeral topic

ploxiln/nsq,ploxiln/nsq,jehiah/nsq,mreiferson/nsq,ploxiln/nsq,rpau/nsq,bitly/nsq,bitly/nsq,bitly/nsq,rpau/nsq,mreiferson/nsq,bitly/nsq,nsqio/nsq,nsqio/nsq,rpau/nsq,rpau/nsq,mreiferson/nsq,nsqio/nsq,jehiah/nsq,jehiah/nsq,ploxiln/nsq,mreiferson/nsq,rpau/nsq,bitly/nsq,jehiah/nsq,nsqio/nsq,jehiah/nsq,ploxiln/nsq,nsqio/nsq,mreiferson/nsq