R1:4d5e8a149a7d

Split Harbormaster workers apart so build steps can run in parallel

Summary:
Ref T1049. Currently, the Harbormaster worker looks like this:

foreach (step) {
run_step(step);
}

This means steps can't ever be run in parallel. Instead, split it into two workers. The "Build" worker starts things off, and basically does:

update_build();

(We could theoretically do this in the original process because it should never take very long, but since there's a lock and it's a little bit complex it seemed…