Acquire exclusive table lock earlier

Dependencies

• [2] MSIHMO45 Tweak build steps
• [3] UQQ4IL55 Add a error type for "unsupported system type"
• [4] IWB3F4Z6 Fail builds with previously failed steps early
• [*] 24BMQDZA Start of single-process hydra-queue-runner

Change contents

• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 604
  [3.810]

  [3.310]
  + createBuildStep(txn, 0, build, r, "", buildStepStatus);
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 611
  [3.1093]→[2.197:269](∅→∅)
  − createBuildStep(txn, 0, build, r, "", buildStepStatus);