In the new world, one builder runs
        gobuilder -commit
which uploads information about commits to the dashboard,
which then hands the work out to the builders by hash.
There is no assumption anymore that the commit numbers
are consistent across builders.

New builders will need to be deployed.  For now darwin-amd64
is running the new builder to test the code.

The new JSON-based protocol for handing out work via /todo
should be easy to extend if we want to add support for sending
trial CLs to the builders.

This code is already running on godashboard.appspot.com.

R=adg, dave
CC=golang-dev
https://golang.org/cl/4519047