@Override
        public String toString() {
            return "ResourceItem{" +
                   "resourceKey=" + resourceKey +
                   ", path=" + path +
                   ", createTime=" + createTime +
                   ", lastAccessTime=" + lastAccessTime +
                   '}';
        }
Delete files that haven't been accessed since the last cleanup.
This allows us to choose the cleanup frequency.
stroom-resource/stroom-resource-impl/src/main/java/stroom/resource/impl/ResourceStoreImpl.java