Wrap the discoverers into separate threads

Fixes #6350 (the regression was introduced with b2241da )
Thanks git-bisect :)

Author: facchinm

arduino-core/src/cc/arduino/packages/Discovery.java

@@ -31,7 +31,7 @@
 
 import java.util.List;
 
-public interface Discovery {
+public interface Discovery extends Runnable {
 
   /**
    * Start discovery service

arduino-core/src/cc/arduino/packages/DiscoveryManager.java

@@ -51,7 +51,7 @@ public DiscoveryManager() {
     // Start all discoverers
     for (Discovery d : discoverers) {
       try {
-        d.start();
+        new Thread(d).start();
       } catch (Exception e) {
         System.err.println(tr("Error starting discovery method: ") + d.getClass());
         e.printStackTrace();

arduino-core/src/cc/arduino/packages/discoverers/NetworkDiscovery.java

@@ -39,7 +39,7 @@
 
 import cc.arduino.packages.discoverers.network.BoardReachabilityFilter;
 
-public class NetworkDiscovery implements Discovery, ServiceListener {
+public class NetworkDiscovery implements Discovery, ServiceListener, Runnable {
 
   private final List<BoardPort> reachableBoardPorts = new LinkedList<>();
   private final List<BoardPort> boardPortsDiscoveredWithJmDNS = new LinkedList<>();
@@ -138,6 +138,11 @@ public NetworkDiscovery() {
 
   }
 
+  @Override
+  public void run() {
+    start();
+  }
+
   @Override
   public void start() {
     jmdns = JmmDNS.Factory.getInstance();

arduino-core/src/cc/arduino/packages/discoverers/SerialDiscovery.java

@@ -37,7 +37,7 @@
 import java.util.List;
 import java.util.Timer;
 
-public class SerialDiscovery implements Discovery {
+public class SerialDiscovery implements Discovery, Runnable {
 
   private Timer serialBoardsListerTimer;
   private final List<BoardPort> serialBoardPorts;
@@ -85,6 +85,11 @@ public void setUploadInProgress(boolean param) {
 
   public void pausePolling(boolean param) { serialBoardsLister.pausePolling = param;}
 
+  @Override
+  public void run() {
+    start();
+  }
+
   @Override
   public void start() {
     this.serialBoardsListerTimer = new Timer(SerialBoardsLister.class.getName());