Close active pconns after their *_port goes away on reconfigure.

Close active pconns after their *_port goes away on reconfigure.

This change reduces what may be perceived as reconfigure memory leaks
related to *_port options. Before this change, a single persistent
connection could continue to receive new requests (and tie no longer
globally accessible PortCfg-related structures) for hours.

TODO: Close already idle pconns as well.