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.

message-queue/README.md

@@ -0,0 +1,14 @@
+# Message Log
+
+A message log for various services to communicate over.
+
+Each message is checked to be of the claimed origin. Then, it's added to the
+recipient's message queue. This queue is sequentially handled, FIFO, only
+dropping messages once the recipient acknowledges it's been handled.
+
+A client which publishes an event specifies its own ID for the publication. If
+multiple publications with the same ID occur, they are assumed repeats and
+dropped.
+
+This library always panics as its error-cases should be unreachable, given its
+intranet status.