Tune nbd backend and io module buffer size

[nirs]

Initial testing with fast NVMe drives show significant speedup when uploading uisng buffer_size of 256k in imageio client and server.

• Run performance tests for upload and download with various buffer sizes.
• Test concurrent upload/download on real server with big images (100g)
• Update server and client defaults
• Document how to run benchmarks in docs/benchmark.md