native-fetcher: bypass the resolver for ipv4 addresses

ipv6 support would be nice too, but would require other code to be
updated to that as well. I created issue
https://github.com/pagespeed/ngx_pagespeed/issues/354 for that

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/351
Merge pull request #355 from pagespeed/oschaaf-native-fetcher-ipaddress-lookup

native-fetcher: bypass the resolver for ipv4 addresses