Adjust for fetchers and options that moved from apache/ to system/ in r3390

Note: temporarily disables the native fetcher, until SystemRewriteDriverFactory is modified to call NewAsyncUrlFetcher() instead of just constructing a Serf fetcher.

`NgxServerContext::ChildInit` is now nearly the same as `ApacheServerContext::ChildInit`, and I want to move them to `SystemServerContext` really soon.