queue: Support sending to pipes

With the introduction of aliases, the queue may now be delivering mail to
pipes. This patch implements pipe delivery.

It uses a fixed 30s timeout for now, as these commands should really not take
much time, and we don't want to overly complicate the configuration for now.

internal/queue/queue.proto

@@ -25,6 +25,7 @@ message Recipient {
 
 	enum Type {
 		EMAIL = 0;
+		PIPE = 1;
 	}
 	Type type = 2;