Fixes for macOS

Dependencies

• [2] W4G5MZZS hydra-evaluator improvements
• [3] K7IOFWLN Fix build
• [4] S2NXJOJE Fix a race that can cause hydra-queue-runner to ignore newly added builds
• [5] HJOEIMLR Refactor
• [6] P6EWEJHL hydra-evaluator: Allow setting the maximum number of concurrent evaluations
• [7] 4YCF3KBG Concurrent hydra-evaluator
• [*] 24BMQDZA Start of single-process hydra-queue-runner

Change contents

• replacement in src/hydra-evaluator/hydra-evaluator.cc at line 9
  [5.692]→[2.75:108](∅→∅)
  − #include <experimental/optional>
  [5.692]

  [5.692]
  + #include <optional>
• replacement in src/hydra-evaluator/hydra-evaluator.cc at line 34
  [5.1102]→[2.167:220](∅→∅)
  − std::experimental::optional<JobsetName> evalOne;
  [5.1102]

  [2.220]
  + std::optional<JobsetName> evalOne;
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 26
  [3.247]

  [3.247]
  + template<> void toJSON<std::atomic<unsigned long long>>(std::ostream & str, const std::atomic<unsigned long long> & n) { str << n; }
• replacement in src/libhydra/db.hh at line 26
  [5.681]→[4.236:289](∅→∅)
  − std::experimental::optional<std::string> status;
  [5.681]

  [4.289]
  + std::optional<std::string> status;
• replacement in src/libhydra/db.hh at line 38
  [4.330]→[4.330:383](∅→∅)
  − std::experimental::optional<std::string> get() {
  [4.330]

  [4.383]
  + std::optional<std::string> get() {
• replacement in src/libhydra/db.hh at line 40
  [4.408]→[4.408:453](∅→∅)
  − status = std::experimental::nullopt;
  [4.408]

  [4.453]
  + status = std::nullopt;