src/main/java/com/googlecode/objectify/cache/EntityMemcache.java
public void empty(final Iterable<Key> keys) {
		final Map<Key, Object> updates = new HashMap<>();

		for (final Key key: keys)
			if (cacheControl.isCacheable(key))
				updates.put(key, null);

		this.memcacheWithRetry.putAll(updates);
	}
public void empty(final Iterable<Key> keys) {
		final Map<Key, Object> updates = new HashMap<>();

		for (final Key key: keys)
			if (cacheControl.isCacheable(key))
				updates.put(key, null);

		this.memcacheWithRetry.putAll(updates);
	}
Revert a set of keys to the empty state.  Will loop on this several times just in case
the memcache write fails - we don't want to leave the cache in a nasty state.