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.)
system-test: infrastructure, host validation

We are going to need some nginx-specific system tests, and this seemed like a
good place to start. This adds two tests, as well as some infrastructure. With

Squash-merge of my #194 and #195
system-test: support blocking rewrites (fixes #199)
Merge pull request #200 from pagespeed/jefftk-blocking-rewrite-key

system-test: support blocking rewrites
Fixed #102: content handler: priority
system-test: clear cache; simpler init

* always run with a clean cache
 * in the future we should structure the tests so they run twice, once with a clean cache and then again with a populated one, but for now a clean cache should catch more bugs
* stop requiring a file cache path for tests; just use
* fix exit codes to match documentation at the top of the file: 1 is a test failure, 2 is bad args

Squash-merge of my #198.
readme: less scary warning
options: default to core filters
Merge pull request #205 from pagespeed/jefftk-rewrite-level

options: default to core filters
system-test: check loopback fetcher works with vhosts on alternate ips
Merge pull request #206 from pagespeed/jefftk-loopback-fetch

system-test: check loopback fetcher works with vhosts on alternate IPs
system-test: port simple tests, all are exact copies from apache
Merge pull request #210 from pagespeed/jefftk-test-combine-css

system-test: port simple tests, all are exact copies from apache
Merge pull request #189 from chaizhenhua/content_handler

Improved content_handler
request-headers: support X-Forwarded-Proto
Merge pull request #208 from pagespeed/jefftk-respect-x-forwarded-proto

request-headers: support X-Forwarded-Proto
trunk-tracking: RewriteDriverFactory now needs UseBeaconResultsInFilters
trunk-tracking: shared_mem_referer_statistics were removed
trunk-tracking: use System and support cache flushing

Use SystemRewriteDriverFactory.
Use SystemServerContext.
Wire up FlushCacheIfNecessary().
Port cache flushing tests.
psol: updating from r2748