Reduce thread pool size for less resource use

src/main/java/org/qortal/network/Network.java

@@ -164,7 +164,7 @@ public class Network {
         maxPeers = Settings.getInstance().getMaxPeers();
 
         // We'll use a cached thread pool but with more aggressive timeout.
-        ExecutorService networkExecutor = new ThreadPoolExecutor(4,
+        ExecutorService networkExecutor = new ThreadPoolExecutor(2,
                 Settings.getInstance().getMaxNetworkThreadPoolSize(),
                 NETWORK_EPC_KEEPALIVE, TimeUnit.SECONDS,
                 new SynchronousQueue<Runnable>(),