BUG/MINOR: server: always count one idle slot for current thread

The idle server connection estimates brought in commit bdb86bd ("MEDIUM:
server: improve estimate of the need for idle connections") were committed
without the minimum of 1 idle conn needed for the current thread. The net
effect is that there are bursts of dropped connections when the load varies
because there's no provision for the last connection.

No backport needed, this is 2.2-dev.
1 file changed