Add a message queue

This is intended to be a reliable transport between the processors and
coordinator. Since it'll be intranet only, it's written as never fail.

Primarily needs testing and a proper ID.

Cargo.toml

@@ -23,6 +23,8 @@ members = [
   "coins/monero/generators",
   "coins/monero",
 
+  "message-queue",
+
   "processor/messages",
   "processor",