back-port #1719 (closed) stats underflow fix to 9.11

A customer has reported seeing:

named.stats:          4294967295 recursing clients
named.stats.old:                   1 recursing clients

Looks like this is underflowing again, even with

5374.   [bug]           Statistics counters tracking recursive clients and
                        active connections could underflow. [GL #1087]

https://support.isc.org/Ticket/Display.html?id=16711