Add a timed join() method to Channel, update Exec example

src/main/java/net/schmizz/sshj/connection/channel/AbstractChannel.java

@@ -272,6 +272,11 @@ public abstract class AbstractChannel
         close.await();
     }
 
+    public void join(int timeout, TimeUnit unit)
+            throws ConnectionException {
+        close.await(timeout, unit);
+    }
+
     protected synchronized void sendClose()
             throws TransportException {
         try {

src/main/java/net/schmizz/sshj/connection/channel/Channel.java

@@ -23,6 +23,7 @@ import net.schmizz.sshj.transport.TransportException;
 import java.io.Closeable;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.util.concurrent.TimeUnit;
 
 /** A channel is the basic medium for application-layer data on top of an SSH transport. */
 public interface Channel
@@ -138,4 +139,7 @@ public interface Channel
     void join()
             throws ConnectionException;
 
+    void join(int timeout, TimeUnit unit)
+            throws ConnectionException;
+
 }