Support Cache Flushing

The current mod_pagespeed trunk moves cache flushing into shared code, so we can use it.

I've added tests, and the "Flushing one cache does not flush all caches" test fails.  This is because kCacheFlushTimestampMs is currently stored in the statistics, and ngx_pagespeed doesn't have per-vhost statistics yet.  I think this is a bug in PSOL that you need per-vhost stats in order to have per-vhost flushing, and I'm trying to fix this internally.