nsqd: reduce ReadyCount update frequency

[andyxning]

This PR will reduce the update frequency for ReadyCount when the new rdy count equals with the old one.

[andyxning]

/cc @ploxiln @mreiferson

[ploxiln]

This results in two atomic operations for the "change" case. It could be just one like:

old_count = atomic.SwapInt64(&c.ReadyCount, count)
if old_count != count {
    c.tryUpdateReadyState()
}

(I suppose the "no change" case happens quite a lot for some of your services? if it's not too frequent, it's not expensive ...)

[andyxning]

@ploxiln Thanks for your good advice.

I suppose the "no change" case happens quite a lot for some of your services? if it's not too frequent, it's not expensive ...

Yes. it happens a lot. But even if it is not, avoiding useless logic is good, right? :)

[ploxiln]

cool. thanks