trunk-tracking: update to r3448 from r3403 

As of r3447, `ServerContext::ChildInit` is now in `system/`, which means we can delete a lot of code.

`DangerFetchFromUnknownHosts` and `FetchWithGzip` are now handled by `RewriteOptions`.

`ServerContext` now needs a hostname and port, but we don't have one so we hackishly provide a different unique id.