malloc: Trim unused arenas on thread exit

[Siddhesh Poyarekar]

On Tuesday 14 November 2017 12:43 PM, Florian Weimer wrote:
> But in these cases, the application will generally use a thread pool, so
> glibc will not see the thread exit anyway.

We kinda emulate thread pools with our thread stack cache and arena
cache; the only thing we don't cache is the actual threads.  If your
premise is that it is not important enough then the premise will likely
extend to the utility of this optimization.

Siddhesh