rabbitmq-server: 3.6.10 -> 3.7.8

- New dependency on 'getconf' binary for
  https://github.com/rabbitmq/rabbitmq-common/blob/3aa619e9ef81cfe6db0c776e6cd829ce234a7881/src/vm_memory_monitor.erl#L448
- New dependency on 'socat' for systemd notifications
  https://github.com/rabbitmq/rabbitmq-server/blob/4a3ee3a336bc82f2e56ec5d2292c80458ccc2974/src/rabbit.erl#L361
- elixir_1_6 for a new 'rabbitmqctl' tool
- Replace patching with providing custom PATH, as we already have some
  other things here
- Renamed package in all-packages.nix from a legacy spelling