mongos hangs waiting for downed config server

If a config server doesn't respond, mongos hangs waiting for the connection. To reproduce, drop incoming config server packets from one of three config servers:

sudo /sbin/iptables -A INPUT -i eth0 -p tcp -m tcp --dport THE_LISTEN_PORT -j DROP

and start a new mongos instance pointed at these servers, should hang.

Killing the config server entirely allows mongos to start.