Use daemon thread to avoid blocking JVM shutdown (#605)

src/main/java/net/schmizz/keepalive/KeepAlive.java

@@ -30,6 +30,7 @@ public abstract class KeepAlive extends Thread {
         this.conn = conn;
         log = conn.getTransport().getConfig().getLoggerFactory().getLogger(getClass());
         setName(name);
+        setDaemon(true);
     }
 
     public synchronized int getKeepAliveInterval() {

src/main/java/net/schmizz/sshj/sftp/PacketReader.java

@@ -42,6 +42,7 @@ public class PacketReader extends Thread {
         log = engine.getLoggerFactory().getLogger(getClass());
         this.in = engine.getSubsystem().getInputStream();
         setName("sftp reader");
+        setDaemon(true);
     }
 
     private void readIntoBuffer(byte[] buf, int off, int len)

src/main/java/net/schmizz/sshj/transport/Reader.java

@@ -30,6 +30,7 @@ public final class Reader
         this.trans = trans;
         log = trans.getConfig().getLoggerFactory().getLogger(getClass());
         setName("reader");
+        setDaemon(true);
     }
 
     @Override