Changes:
* r3585: With downstream caching, don't touch `Cache-Control` headers.
* No longer require `Modify Caching Headers off`.
* Change from `modify_caching_headers` as a boolean to a three valued enum
`PreserveCachingHeaders`.
* r3596: Make tests less flaky.
* Changes WGET_DUMP to write to WGET_DIR instead of OUTDIR.
* r3597: Remove now-redundant system tests.
* Some system tests were moved into `automatic/system_test.sh` which means we
can remove our forks.
* r3598: Enable the shared memory metadata cache by default.
* The code change just worked and needed no changes, but it also added
substantial tests, which needed some porting.
* Deflake the `scrape stats` test by moving it before config reloading.
Jeff Kaufman
07286005a6