Add repro for bitwalker/swarm#91

Dockerfile

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+FROM elixir:1.6.6-alpine
+
+RUN mkdir /app
+WORKDIR /app
+COPY . .
+RUN mix do local.hex --force, deps.get, compile

README.md

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+### Swarm deadlock repro
+
+This repository serves as a repro for bitwalker/swarm#91 .
+To start reproducing, simply run:
+
+```sh
+docker-compose up --scale repro=5
+```
+
+This command will build a docker image with the compiled repro Elixir application,
+and run 5 instances of the image. All the application does is start `Swarm` and
+ `Libcluster` with `Cluster.Strategy.Gossip` topology. A single process on each
+node prints state of the local `Swarm.Tracker` process.

docker-compose.yml

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+version: '2'
+services:
+  repro:
+    build: .
+    command: ['sh', '-c', 'iex --cookie mycookie --name "node@$$(hostname -i)" -S mix']

lib/my_app.ex

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+defmodule TrackerStateMonitor do
+  use Task, restart: :permanent
+
+  def start_link(_args) do
+    Task.start_link(fn ->
+      Stream.repeatedly(fn -> Process.sleep(5000) end)
+      |> Enum.each(fn _ ->
+        {state, _} = :sys.get_state(Swarm.Tracker)
+        IO.puts("Swarm.Tracker state: #{state}")
+      end)
+    end)
+  end
+end
+
+defmodule MyApp do
+  use Application
+
+  @impl Application
+  def start(_type, _args) do
+    Supervisor.start_link([
+      {Cluster.Supervisor,
+       [[local: [strategy: Cluster.Strategy.Gossip]], [name: MyApp.ClusterSupervisor]]},
+      {TrackerStateMonitor, []}
+    ], strategy: :one_for_one)
+  end
+end

mix.exs

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+defmodule MyApp.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :my_app,
+      version: "0.1.0",
+      elixir: "~> 1.6",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger, :sasl],
+      mod: {MyApp, []}
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      {:libcluster, "~> 3.0"},
+      {:swarm, "~> 3.3"}
+    ]
+  end
+end