malloc: Trim unused arenas on thread exit

[Siddhesh Poyarekar]

On Wednesday 15 November 2017 04:14 PM, Florian Weimer wrote:
> It could be related to the ordering between tcache_thread_freeres and
> arena_thread_freeres.  We should merge the two to enforce the correct
> ordering.

I agree, it sounds like a better idea to have a single freeres for a
module anyway.

Siddhesh