trunk-tracking: update from r3715 to r3736

Squash-merge of Jan's #608 and Otto's #611.

* r3726:
  * Updated closure compiler flags for static JS files.
* r3729:
  * Centralize parsing of FetchHttps in SystemRewriteOptions so ngx_pagespeed
    can get it too.
  * To keep the helpful error_message from SerfUrlAsyncFetcher, wire it through
    RewriteOptions as a new-fangled error_detail.
* r3735:
  * Follow-up changes for downstream caching integration with beaconing
    dependent filters: If a downstream cache rebeaconing key is configured, we
    should instrument the page only if the key present in the PS-ShouldBeacon
    header matches the one in the configuration. This allows us to send no-cache
    headers for anything that carries the right beaconing key, and continue to
    send out the original cache control headers in other cases where downstream
    caching is enabled.
* Native fetcher: fortify handling of content length (and absense).
* Native fetcher: fail when the stream terminates before having
  completely parsed the headers.
* Tests: Rename `test_filter` -> `start_test` in ngx_system_test.sh for
  a test.
* Tests: Move blockingrewrite key to the http {} block.
* Tests: Update localhost -> 127.0.0.1. The native fetcher uses
  dns to resolve, and won't be able to retreive an ip for
  localhost.
* Tests: Allow outstanding proxy fetches some time to finish
  when running under valgrind, before terminating nginx.
* Valgrind: Add suppressions to make testing a release build pass.

This pull update was extra work because the valgrind and native fetcher flows
had rotted a bit.  We need to make sure to test them with every update.