Resolve #360 (#456)

* Remove NetworkId from processor-messages

Because intent binds to the sender/receiver, it's not needed for intent.

The processor knows what the network is.

The coordinator knows which to use because it's sending this message to the
processor for that network.

Also removes the unused zeroize.

* ProcessorMessage::Completed use Session instead of key

* Move SubstrateSignId to Session

* Finish replacing key with session

message-queue/src/main.rs

@@ -69,12 +69,7 @@ mod binaries {
     // Assert one, and only one of these, is the coordinator
     assert!(matches!(meta.from, Service::Coordinator) ^ matches!(meta.to, Service::Coordinator));
 
-    // Verify (from, intent) hasn't been prior seen
-    // At the time of writing, intents should be unique even across `from`. There's a DoS where
-    // a service sends another service's intent, causing the other service to have their message
-    // dropped though.
-    // Including from prevents that DoS, and allows simplifying intents to solely unique within
-    // a service (not within all of Serai).
+    // Verify (from, to, intent) hasn't been prior seen
     fn key(domain: &'static [u8], key: impl AsRef<[u8]>) -> Vec<u8> {
       [&[u8::try_from(domain.len()).unwrap()], domain, key.as_ref()].concat()
     }