Fix finishing steps that are not top-level of any build

Dependencies

• [2] HUUZFPPK Fix race between the queue monitor and the builder threads
• [3] ENXUSMSV Make concurrency more robust
• [4] OCZ4LSGG Automatically retry aborted builds
• [*] 24BMQDZA Start of single-process hydra-queue-runner

Change contents

• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 1122
  [2.3946]→[2.3946:4003](∅→∅)
  − then we're done. Delete the step from
  [2.3946]

  [2.4003]
  + then we're done (except for calling

  + finishBuildStep()). Delete the step from
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 1131
  [2.4421]→[2.4421:4448](∅→∅)
  − break;
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 1147
  [6.16116]

  [2.4907]
  + if (direct.empty()) break;

  +