Don't count idle connections against maxTotal

[rossabaker]

No description provided.

[rossabaker]

We don't need a permit unless we're allocating a resource. When we do, we don't want to release the permit if we're resuing it. bada1fa does the trick, but exposes a different problem: if we're at maxTotal between all the idle connections, we need to release at least one to make room for an acquisition.

Prior art:

• commons-pool2 evicts the oldest 15% and tries again. This is non-trivial to calculate, and might not be a good fit for a ref.update.
• blaze drops a random object and tries again.