Merge remote-tracking branch 'origin/master' into flake

Dependencies

• [2] P3M6UFMP Merge remote-tracking branch 'origin/master' into flake
• [3] TF5HAPRD export a /prometheus endpoint
• [4] VEXG3QPK hydra: upgrade nixpkgs to 19.09
• [5] QPDG4NEU release.nix: Use fetchGit to get Nixpkgs
• [6] BGMBEXS6 Gradually increase maxHeapSize
• [7] P33UMMOO Use nixUnstable
• [8] AVGER75N release: bump to 19.03
• [9] EQ3F427Z also add CatalystXRoleApplicator
• [10] FJ5H26J6 Fix building against nix master
• [11] ICYJNJWH Revert "Move build logic for the core hydra package into its own default.nix."
• [12] R5V57UKM flake.nix improvements
• [13] YQWH4POV * Simplify.
• [14] T4LLYESZ * Nix expression for building Hydra.
• [15] IE6AMEHD fix hydra build on nixpkgs master

Change contents

• edit in flake.nix at line 80
  [2.3212]

  [2.3212]
  + NetPrometheus
• edit in flake.nix at line 343
  [5.35]→[4.0:114](∅→∅),[5.35]→[4.0:114](∅→∅),[5.1793]→[3.0:26](∅→∅),[5.1793]→[3.0:26](∅→∅)
  − , nixpkgs ? builtins.fetchGit { url = https://github.com/NixOS/nixpkgs-channels.git; ref = "nixos-19.09-small"; }

  − NetPrometheus
• resolve order conflict in flake.nix at line 343
  [2.10590]