Chunk server: replace ostringstream with AppendDecIntToString() when creating chunk file names to reduce number of memory allocations and cpu utilization. Pass chunk files with invalid headers and read errors from chunk directory checker to chunk manager. Handle "invalid" chunk files in chunk manager by moving such chunk files into "lost+found" directory, if configured. Maintain chunk directory check interval independently of chunk directories check thread wake up, in order to allow to wake up thread to close lock files with minimal latency. Change chunk files random sampling logic from selecting a fixed portion ("percentage") of chunk files to a configurable fixed number of chunk files sampled (default 16) in order to minimize the chunk directory [initial] load time, and disk io with large number of chunk files.