Fix build failure on GCC 5.4

Dependencies

• [2] BYVRA54Q Temporarily disable machines on any exception, not just connection failures
• [3] MHVIT4JY Split hydra-queue-runner.cc more
• [4] YR2IM6Y5 Temporarily disable machines after a connection failure
• [5] 46ADBTMQ Start steps in order of ascending build ID
• [6] UPNGFCXG For completeness, re-implement meta.schedulingPriority
• [7] FITVNQ2S Keep track of the time we spend copying to/from build machines
• [8] 4I2HF4L3 Unindent
• [9] NKQOEVVP Get rid of "will retry" messages after "maybe cancelling..."
• [10] LE4VZIY5 More stats
• [*] 5AIYUMTB Basic remote building

Change contents

• edit in src/hydra-queue-runner/build-remote.cc at line 2
  [12.107]

  [12.107]
  + #include <cmath>
• replacement in src/hydra-queue-runner/build-remote.cc at line 385
  [2.9873]→[2.9873:9980](∅→∅)
  − int delta = retryInterval * powf(retryBackoff, info->consecutiveFailures - 1) + (rand() % 30);
  [2.9873]

  [2.9980]
  + int delta = retryInterval * std::pow(retryBackoff, info->consecutiveFailures - 1) + (rand() % 30);
• replacement in src/hydra-queue-runner/builder.cc at line 41
  [3.1163]→[3.1118:1212](∅→∅)
  − int delta = retryInterval * powf(retryBackoff, step_->tries - 1) + (rand() % 10);
  [3.1163]

  [3.1241]
  + int delta = retryInterval * std::pow(retryBackoff, step_->tries - 1) + (rand() % 10);
• edit in src/hydra-queue-runner/dispatcher.cc at line 2
  [3.14650]

  [3.14650]
  + #include <cmath>
• replacement in src/hydra-queue-runner/dispatcher.cc at line 123
  [3.2219]→[3.2219:2369](∅→∅)
  − float ta = roundf(a.currentJobs / a.machine->speedFactor);

  − float tb = roundf(b.currentJobs / b.machine->speedFactor);
  [3.2219]

  [3.2369]
  + float ta = std::round(a.currentJobs / a.machine->speedFactor);

  + float tb = std::round(b.currentJobs / b.machine->speedFactor);