Fix building against nix master

Dependencies

• [2] G7MH6KS7 Add boost dependency
• [3] BGMBEXS6 Gradually increase maxHeapSize
• [4] DKJFD6JN Process Nix API changes
• [5] EJJCY263 Account steps with preferLocalBuild as a separate system type
• [6] MHVIT4JY Split hydra-queue-runner.cc more
• [7] P33UMMOO Use nixUnstable
• [8] EFWDY2V7 Support testing build determinism
• [9] HJOEIMLR Refactor
• [10] T4LLYESZ * Nix expression for building Hydra.
• [11] SL3WSRAC hydra-queue-runner: Limit memory usage
• [12] QPDG4NEU release.nix: Use fetchGit to get Nixpkgs
• [13] YQWH4POV * Simplify.
• [14] H7SZRHUB Use nix::willBuildLocally()
• [*] ICYJNJWH Revert "Move build logic for the core hydra package into its own default.nix."
• [*] OG3Z3QGC Namespace cleanup

Change contents

• replacement in release.nix at line 2
  [4.35]→[3.0:114](∅→∅)
  − , nixpkgs ? builtins.fetchGit { url = https://github.com/NixOS/nixpkgs-channels.git; ref = "nixos-18.03-small"; }
  [4.35]

  [4.65]
  + , nixpkgs ? builtins.fetchGit { url = https://github.com/NixOS/nixpkgs-channels.git; ref = "nixos-18.09-small"; }
• edit in release.nix at line 114
  [2.16]

  [16.2651]
  + nlohmann_json
• edit in src/hydra-queue-runner/queue-monitor.cc at line 431
  [4.31444]

  [4.0]
  + step->parsedDrv = std::make_unique<ParsedDerivation>(drvPath, step->drv);
• replacement in src/hydra-queue-runner/queue-monitor.cc at line 433
  [4.1]→[4.950:1009](∅→∅)
  − step->preferLocalBuild = step->drv.willBuildLocally();
  [4.1]

  [4.657]
  + step->preferLocalBuild = step->parsedDrv->willBuildLocally();
• replacement in src/hydra-queue-runner/state.hh at line 13
  [4.1521]→[4.1521:1547](∅→∅)
  − #include "derivations.hh"
  [4.1521]

  [4.1299]
  + #include "parsed-derivations.hh"
• edit in src/hydra-queue-runner/state.hh at line 155
  [17.403]

  [4.2411]
  + std::unique_ptr<nix::ParsedDerivation> parsedDrv;