Add support to client transports for requests that may result in multiple

responses. (#377)

This adds ComposeRequestMulti and other *Multi types. The main change is to
the stream transport, which is the only transport that implements
SendRequestMulti.

src/base/message.rs

@@ -439,6 +439,14 @@ impl<Octs: Octets + ?Sized> Message<Octs> {
         }
     }
 
+    /// Returns whether the message has a question that is either AXFR or
+    /// IXFR.
+    pub fn is_xfr(&self) -> bool {
+        self.first_question()
+            .map(|q| matches!(q.qtype(), Rtype::AXFR | Rtype::IXFR))
+            .unwrap_or_default()
+    }
+
     /// Returns the first question, if there is any.
     ///
     /// The method will return `None` both if there are no questions or if