Nydusd benchmark

1. Install Nix on a Linux box. You don't need NixOS.
2. Run ./build-and-run-vm.sh to launch a VM running nydus snapshotter, containerd, and minio set up with 30ms network delay between minio and containerd.
3. Inside the VM, log in with nydus as username and password as password.
4. Run create-container to create a Nydus image with many small files and a few large ones, and write it to a local registry.
5. Run the various benchmark-* scripts to run access speed benchmarks.
6. To kill the VM, run pkill qemu from another terminal (though be careful, it might kill other VMs too).
7. If you want to reset the VM state (you'll have to run create-container again), delete the .qcow2 files created in PWD.