converter must be created when required (problem with concurrent access)

rest-api/src/main/java/lcsb/mapviewer/api/BaseRestImpl.java

@@ -311,7 +311,11 @@ public abstract class BaseRestImpl {
   protected Converter getModelParser(String handlerClass) throws QueryException {
     for (Converter converter : getModelConverters()) {
       if (converter.getClass().getCanonicalName().equals(handlerClass)) {
-        return converter;
+        try {
+          return (Converter) Class.forName(handlerClass).newInstance();
+        } catch (InstantiationException | IllegalAccessException | ClassNotFoundException e) {
+          throw new QueryException("Problem with handler:" + handlerClass);
+        }
       }
     }
     throw new QueryException("Unknown handlerClass: " + handlerClass);