Add property cache

Add support for the property cache by creating a property cache callback and passing to StartNewProxyFetch. With this change the insert_dns_prefetch test passes.

This implementation mostly duplicates several functions in proxy_interface.cc, which is unfortunate. There isn't an instantiation though that I can find of ProxyInterface in ngx_pagespeed, and I'm not sure where the best place to add one would be. Comments on that are welcome.