native-fetcher: timeouts

After more testing, it turns out that the timer that is initially set
in Init() remains active throughout the whole lifetime of the fetch.

This reverts the previous change, which isn't correct, and replaces
all TODO's concerning timers with a comment about that we don't need
them.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/466
Merge pull request #472 from pagespeed/oschaaf-native-fetcher-timeouts

native-fetcher: timeouts