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
trunk-tracking: built relase binaries off of r2748
trunk-tracking: built debug binaries off of r2748
trunk-tracking: document that we're now on r2748