public Builder diskUsage(DiskUsage diskUsage) {
            this.diskUsage = checkNotNull(diskUsage);
            return this;
        }
Set custom DiskUsage logic for handling when to keep or clean cache.

@param diskUsage a disk usage strategy, cant be {@code null}.
@return a builder.
VideoCache/src/main/java/com/yc/videocache/HttpProxyCacheServer.java