requests could simultaneously result in a load(). Only requests that
enter singleflight Do concurrently would be deduped, so it was possible for
populateCache to be called multiple times for the same key. That
would result in overcounting nbytes, and eventually leading to a
livelock where nbytes > cacheBytes, but there were no entries in the
cache.
Change-Id: I5b304ce82041c1310c31b662486744e86509cc53
