EXIT when node is shut down

[harunzengin]

We get the following EXIT when a node is shut down in v0.18.0-rc.2:

** (EXIT from #PID<0.214.0>) shell process exited with reason: an exception was raised:
    ** (ArgumentError) errors were found at the given arguments:

  * 1st argument: not a reference

        (erts 14.0.2) :erlang.demonitor(nil, [:flush])
        (xandra 0.18.0-rc.2) lib/xandra/cluster/pool.ex:561: Xandra.Cluster.Pool.stop_pool/2
        (xandra 0.18.0-rc.2) lib/xandra/cluster/pool.ex:308: Xandra.Cluster.Pool.handle_event/4
        (stdlib 5.0.2) gen_statem.erl:1379: :gen_statem.loop_state_callback/11
        (stdlib 5.0.2) proc_lib.erl:241: :proc_lib.init_p_do_apply/3