Add a mechanism (or document how) to pre-warm the cache

There seem to be no way to pre-warm the contents of groupcache at startup. My use-case is described in https://groups.google.com/forum/#!topic/golang-nuts/MweGX6QWNyY but, briefly, is to make sure that after startup my application can process at full speed (this requires groupcache to be fully warmed up) instead of taking 20+ minutes for the empty cache to warm up, during which processing is very slow.