fit(client): cancel blocking DNS lookup if GaiFuture dropped (#2689)

Closes #2686

src/client/connect/dns.rs

@@ -159,6 +159,12 @@ impl fmt::Debug for GaiFuture {
     }
 }
 
+impl Drop for GaiFuture {
+    fn drop(&mut self) {
+        self.inner.abort();
+    }
+}
+
 impl Iterator for GaiAddrs {
     type Item = SocketAddr;