nsqd: start Topic messagePump separately from create

[ploxiln]

with new Topic.Start()

and make Topic.Pause() and Topic.GetChannel() no longer block
on the notification channels for Topic.messagePump()

and simplify some calls to WaitGroupWrapper.Wrap()

follow-up to #1050 cc @mreiferson
possibly better, but does change the behavior of Pause() and GetChannel() slightly ...

[mreiferson]

I like the way this is coming out, thanks.

and make Topic.Pause() and Topic.GetChannel() no longer block on the notification channels for Topic.messagePump()

I was wondering if this would cause random timing-related test failures, looks like it hasn't. However, I'm a little concerned about the subtlety of this change.

For Pause(), I'm thinking that for topic creation code paths, GetTopic and NewTopic should really take paused as a parameter in their signatures. This way we wouldn't have to call Paused immediately after to set appropriate state (code would look a _little_cleaner too, I think). I realize we'll have to change 1000 places in tests we call GetTopic, but I think I'd prefer that over changing the behavior of the notification.

I don't have any great ideas for GetChannel though, what do you think?

[ploxiln]

I don't like the idea of adding a paused parameter to GetTopic() (only to be used if it creates a new topic?)

However, I came up with another design. Topic.messagePump() is started by NewTopic() as before, but now it has a pre-start-loop to recv on pauseChan and channelUpdateChan, and which is ended via a new startChan.

[mreiferson]

Nice, I like it. LGTM, squash?

[ploxiln]

Will squash and do some local testing ...

[ploxiln]

updated

[ploxiln]

thanks for the review!