aliases: Implement "via" aliases

This patch implements "via" aliases, which let us explicitly select a
server to use for delivery.

This feature is useful in different scenarios, such as a secondary MX
server that forwards all incoming email to a primary.

For now, it is experimental and the syntax and semantics are subject to
change.

internal/queue/queue.proto

@@ -27,6 +27,7 @@ message Recipient {
 	enum Type {
 		EMAIL = 0;
 		PIPE = 1;
+                FORWARD = 2;
 	}
 	Type type = 2;
 
@@ -43,6 +44,9 @@ message Recipient {
 	// This is before expanding aliases and only used in very particular
 	// cases.
 	string original_address = 5;
+
+        // The list of servers to use, for recipients of type == FORWARD.
+        repeated string via = 6;
 }
 
 // Timestamp representation, for convenience.