Import IScheduler instead of Scheduler in DhtHandshake class

`DhtHandshake` only needs the interface, rather than access to the full
`Scheduler` class.  This was not possible in dhtproto v13.x.x since its
minimum ocean dependency predated introduction of the interface, but we
can make this change in v14.x.x.

src/dhtproto/client/legacy/internal/helper/Handshake.d

@@ -17,7 +17,7 @@ public class DhtHandshake
     import dhtproto.client.DhtClient;
     import dhtproto.client.legacy.internal.helper.RetryHandshake;
 
-    import ocean.task.Scheduler;
+    import ocean.task.IScheduler;
     import ocean.task.Task;
     import ocean.task.util.Event;
     import ocean.util.log.Logger;