trunk-tracking: update to 1.7.30.1 from r3581

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.