Use DynamicSupervisor for on_demand mode.

[maltoe]

relates to #233

[maltoe]

@andreasknoepfle Please :eyes: this only when you have a lot of time. It's not urgent.

In fact, I'm second guessing myself:

pid = spawn(fn ->
  try do
    :timer.sleep(1000)
  after
    IO.puts("never executes")
  end
end)

Process.exit(pid, :kill)
:timer.sleep(1000)

When the client process exits, the after is never executed and hence the DynamicSupervisor child will not be terminated :scream:

[maltoe]

Yeah, nevermind this.