Benchmarks: Re-factored benchmark infra

Major re-factoring to use a dual-channel strategy for executing
benchmarks. Uses cluster metadata for managing lifecycle events, but
transport channel to send benchmark definitions and results between
master and executor nodes.
if you use `NOCOMMIT` instead of `XXX` our builds catch that but the build will fail. I can make sure bill switches off the validation on the feature branches! I will look into that later and will let you know. if you build `-Dvalidate.skip=true` it will be cool