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.)

scripts/copy_includes.sh

@@ -58,6 +58,8 @@ rsync -arvz "$MOD_PAGESPEED_SRC/" "psol/include/" --prune-empty-dirs \
   --include="apr_mem_cache.cc" \
   --include="key_value_codec.cc" \
   --include="apr_memcache2.c" \
+  --include="loopback_route_fetcher.cc" \
+  --include="add_headers_fetcher.cc" \
   --exclude='*'
 git add psol/include/