trigger garbage collection periodically

This reduces the memory usage, because the old generation can be made
smaller and we don't have to wait until max heap usage has been reached.

pdb-ui/src/main/java/org/lucares/pdbui/PdbWebapp.java

@@ -1,10 +1,29 @@
 package org.lucares.pdbui;
 
+import java.util.concurrent.TimeUnit;
+
 import org.springframework.boot.SpringApplication;
 
 public class PdbWebapp {
 
 	public static void main(final String[] args) throws Exception {
 		SpringApplication.run(MySpringConfiguration.class, args);
+		
+		Thread t = new Thread(()-> {
+			
+			
+			while(true){
+				try{
+					TimeUnit.MINUTES.sleep(10);
+				}catch(InterruptedException e)
+				{
+					// ignore
+				}
+				System.gc();
+			}
+		});
+		t.setDaemon(true);
+		t.setName("periodic-gc");
+		t.start();
 	}
 }