Improved connection draining with small number of connections.

Improved connection draining with small number of connections.

Closing up to 32 connections might be too aggressive if worker_connections
is set to a comparable number (and/or there are only a small number of
reusable connections).  If an occasional connection shorage happens in
such a configuration, it leads to closing all reusable connections instead
of gradually reducing keepalive timeout to a smaller value.  To improve
granularity in such configurations we now close no more than 1/8 of all
reusable connections at once.

Suggested by Joel Cunningham.