Promptly freeing the per-thread cached direct buffers when a thread exits

[Florian Weimer]

* Tony Printezis:

> There are a few obvious ways to mitigate this, e.g., cause a Full GC /
> concurrent GC cycle at regular intervals. However, the best solution IMHO
> is to explicitly free any direct buffers that are still in the cache when a
> thread exits.

Why is this safe?  Couldn't these direct byte buffers be used with a
custom channel that leaks them to another thread?