make 'connect_timeout' configurable

I make use of the [connect_timeout](https://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_connect_timeout) system variable in many environments, usually to deal with what I call 'optimistic networking' code.. where a developer assumes the socket isn't going away from the front side of a load balancer.  I've set the default value (10) to match the default provided by MySQL so it shouldn't affect behaviour in anyone's environment unless they'd with it to.

I believe this system variable has been in MySQL for a long time, so I don't think there's any harm in assuming we can set it.