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().
trunk-tracking: test cache flushing works
With per-vhost stats we can flush just one cache