limit the number of goroutines

[achille-roussel]

@f2prateek

Following up on our conversation, this should limit the number of goroutines spawned by the client and block if too many are in use already.

[f2prateek]

Neat, haven't used sync.Cond before. I think we can simplify by using a semaphore directly though https://github.com/f2prateek/semaphore if we want to simplify.

LGTM!

[achille-roussel]

Yep, a semaphore primitive would be great to have in Go... I don't know if Go optimizes allocation of make(chan struct, 1000) to not allocate 1KB+ of memory for the semaphore, if it does it's great, if not I'd rather use sync.Cond + counter, which seems like a more efficient way to manage the semaphore.

The Channel has the advantage of being injectable in a select, I'll probably do that in 3.0.0 to wait for c.quit as well.