Implement NgxServerContext::ApplySessionFetchers

Makes sure we use the LoopbackRouteFetcher, and also wires up AddHeadersFetcher.

Note that this pulls in  loopback_route_fetcher.cc from svn revision r2649

The LoopbackRouteFetcher is applied unconditionally, while in mod_pagespeed it is not applied when one of these configuration settings is true:
- disable_loopback_routing
- slurping_enabled, or
- test_proxy is set

I added a TODO for that.