Keep track of how many threads are waiting

Dependencies

• [2] ZTVVUMTK JSONObject doesn't handle 64-bit integers
• [3] N4IROACV Move buildRemote() into State
• [4] CNLNT3T4 Allow only 1 thread to send a closure to a given machine at the same time
• [5] OCZ4LSGG Automatically retry aborted builds
• [6] LE4VZIY5 More stats
• [7] A2GL5FOZ Moar stats
• [8] 5AIYUMTB Basic remote building
• [*] 24BMQDZA Start of single-process hydra-queue-runner
• [*] HJOEIMLR Refactor
• [*] DODOGD7M Send queue runner stats to statsd

Change contents

• replacement in src/hydra-queue-runner/build-remote.cc at line 182
  [3.873]→[3.873:989](∅→∅),[5.594]→[5.161:205](∅→∅),[3.989]→[5.161:205](∅→∅),[5.161]→[5.161:205](∅→∅)
  − printMsg(lvlDebug, format("sending closure of ‘%1%’ to ‘%2%’") % step->drvPath % machine->sshName);

  − MaintainCount mc(nrStepsCopyingTo);
  [3.873]

  [4.26]
  + auto mc1 = std::make_shared<MaintainCount>(nrStepsWaiting);
• edit in src/hydra-queue-runner/build-remote.cc at line 184
  [4.98]

  [4.98]
  + mc1.reset();

  + MaintainCount mc2(nrStepsCopyingTo);

  + printMsg(lvlDebug, format("sending closure of ‘%1%’ to ‘%2%’") % step->drvPath % machine->sshName);
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 1286
  [5.818]

  [2.0]
  + root.attr("nrStepsWaiting", nrStepsWaiting);
• edit in src/hydra-queue-runner/state.hh at line 206
  [11.5760]

  [11.5760]
  + counter nrStepsWaiting{0};
• edit in src/script/hydra-send-stats at line 32
  [5.1045]

  [12.1059]
  + gauge("hydra.queue.steps.waiting", $json->{nrStepsWaiting});