After @neillm found the host pool div by zero bug I started looking closely at an issue whereby I could never quite reach a targeted number of connections. @pmienk looked at it with me for a few hours last week. It was finally determined that it was the result of a race in the connector class. Despite being stopped (and fully stranded) it could still be running until handlers were executed. Since it was reused, this could cause state corruption despite no actual concurrency (by interleaving). I resolved this today, and now am no longer losing an occasional connector.
I did some work on the address pool, though I'm not really happy with that yet. That's really the major performance issue in terms of connection speed at this point. Also got rid of another boost dependency.