device: add latency and throughput benchmarks

device: add latency and throughput benchmarks

These obviously don't perfectly capture real world performance,
in which syscalls and network links have a significant impact.
Nevertheless, they capture some of the internal performance factors,
and they're easy and convenient to work with.

Hat tip to Avery Pennarun for help designing the throughput benchmark.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>