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.
Merge pull request #494 from pagespeed/jefftk-update-trunk-tracking

trunk-tracking: update to r3448 from r3403