Fix DistributedForwarder State when running multiple nodes

[scudelletti]

Context:

I was trying to setup Commanded with Horde and EventStore and found that something the DistributedForwarder state was empty in some cases, raising an error such as:

14:27:57.931 [error] GenServer Integration.EventStore.EventStore.Registration.DistributedForwarder terminating
** (FunctionClauseError) no function clause matching in :elixir_aliases.do_concat/2
    (elixir) src/elixir_aliases.erl:130: :elixir_aliases.do_concat([[], EventStore.PubSub], "Elixir")
    (elixir) src/elixir_aliases.erl:118: :elixir_aliases.concat/1
    (eventstore) lib/event_store/registration/local_registry.ex:52: EventStore.Registration.LocalRegistry.broadcast/3
    (eventstore) lib/event_store/registration/distributed_forwarder.ex:36: EventStore.Registration.DistributedForwarder.handle_info/2
    (stdlib) gen_server.erl:637: :gen_server.try_dispatch/4
    (stdlib) gen_server.erl:711: :gen_server.handle_msg/6
    (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

This happens when I try to connect 3 nodes following this steps:

1. Start Node A
2. Start Node B
3. Start Node C
4. In Node C connect to Node A and B.
5. In Node C dispatch a command.

Here's a repo with a sample app that is able to reproduce the problem, the app is using SwarmRegistry:
https://github.com/scudelletti/eventstore-error

Reasoning behind the change:

Debugging it I found that the init/1 function initializes the state with an empty list. Setting the provided event_store argument seems to fix the problem.
I'm not sure where the state is set if the init/1 function always set it to empty though.

Others

I can try to provide a sample app with the problem if needed, just please let me know.

@slashdotdash thank you for maintaining Commanded and its related libraries. 🎉 💐 ❤️

[slashdotdash]

Thanks for fixing this issue.