DummyOrdered delivery protocol

[tomusdrw]

Write a pallet that allows queueing outbound messages and is able to receive inbound messages. The messages should have a simple format defined below and the dispatcher should be pluggable.

struct Message {
  lane_id: [u8; 3],
  nonce: u64,
  payload: Vec<u8>,
}

• Messages from each lane should be submitted separately (initially it's fine to mix them up though)
• Initially we will support only one lane.

To receive messages the pallet needs to keep track of:
Map<LaneId, Nonce> to make sure messages are not replayed and processed in order.