[DNS recovery]: Bug fixes

 - we need to unref an interval otherwise its a breaking change. Previously statsd would not keep a process open.
 - we need to reset writePos, we forgot to do so and it's a bug that causes us to flush empty buffers accidentally.

cc @jcorbin @sh1mmer @Matt-Esch @kriskowal
if the default is null, shouldn't this value be reset to null as well?