optimize channel closing to avoid allocations when flushing pending responses

[fwbrasil]

Problem

Scala's foreach is quite difficult for the JIT to optimize. The use of shared abstract implementations in Scala's collections makes call sides become highly polymorphic and simple cases like this one end up requiring allocations and interface dispatches.

Solution

Use a while since pendingResponses is a mutable queue and can efficiently dequeue elements.

Notes

• This is part of Reduce allocation rate #3552
• This issue isn't present in my benchmark implementation anymore because I've fixed the nginx config to use keepalive but I thought it was still worth sending the patch.

[adamw]

I know it's a nit-pick, but maybe pendingResponses.dequeue().apply() would be more readable, as the double ()() look a bit like a typo?

[fwbrasil]

good point. I've just updated it