loopback-routing: apply session fetchers

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.

(Sqash-merge of Otto's #193.)