Log Message:
Update net/rabbitmq to 3.6.0.
Clean up and simplify Makefile.

Breaking changes in 3.6.0:

- Minimum required Erlang version is R16B03 for plain ("just TCP")
  connections for all protocols and 17.5 for TLS ones (18.x is
  recommended for both).
- .NET client now requires .NET 4.5.
- "Immediate" flag is removed from the .NET client (it hasn't been
  supported by the server since RabbitMQ 3.0).
- Default subscription TTL in MQTT is now 24 hours.
- Server artifacts are now distributed as xz archives and not gz.
- Build system has been completely reworked and now uses erlang.mk.
  3rd party plugins must be adapted to the new build system.

Key improvements in this release are:

- Lazy queues
- Much better queue synchronisation throughput
- Lower RAM use, tunable flow control
- Stronger password encryption with pluggable algorithms
- Development moved to GitHub; build system now uses erlang.mk
- Significant improvements to Web STOMP
- Experimental WinRT-compatible .NET client, SQL CLR compatibility
  in the "regular" one
- Pagination in management UI
- More popular plugins now ship with the broker: rabbitmq_sharding
  and rabbitmq_event_exchange, for example.

Full release notes:

  https://github.com/rabbitmq/rabbitmq-server/releases/tag/rabbitmq_v3_6_0