Change DNS query trigger to timer event basis

[nicholaschiasson]

This module would really be more clean if the trigger for DNS query was on a timer event. That way, all jdomain DNS would be self updating and not require any traffic to an upstream just in order to keep it up to date with the DNS record.

[splitice]

In some cases the current method is actually ideal as it doesnt require locks or shared memory (or work per worker for low frequency blocks).