native-fetcher: fix CHECK failure on DNS timeout

Make sure we clear the timeout event associated with resolving
a hostname in `CallbackDone()`, which would otherwise end up
calling `CallbackDone()` a second time upon firing.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/625
Merge pull request #655 from pagespeed/oschaaf-native-fetcher-resolve-timeout

native-fetcher: fix CHECK failure on DNS timeout