malloc: Trim unused arenas on thread exit

[DJ Delorie]

Florian Weimer <fweimer@redhat.com> writes:
> I think trimming unused arenas as much as possible is a good heuristics 
> to minimize RSS, so getting this to work might be worthwhile.

I ran this against many of my real-world workloads and did not see a
significant (i.e. obvious) reduction in actual RSS.  Some tests showed a
minor (i.e. untrustworthy, I didn't run enough passes to reduce the
noise below any measured change) *increase* in RSS, as well as an
expected (but still untrustworthy) increase in cycles required.

Now that we have the tools to *measure* what our malloc changes cause, I
think it's prudent that we start asking for benchmarks (and workloads)
that back up any proposed performance/rss change.