fix for solr shutdown

source/net/yacy/cora/services/federated/solr/MultipleSolrConnector.java

@@ -72,6 +72,7 @@ public class MultipleSolrConnector implements SolrConnector {
 
     @Override
     public void close() {
+        // send termination signal to worker
         for (@SuppressWarnings("unused") AddWorker element : this.worker) {
             try {
                 this.queue.put(POISON_DOC);
@@ -79,6 +80,14 @@ public class MultipleSolrConnector implements SolrConnector {
                 e.printStackTrace();
             }
         }
+        // wait for termination
+        for (AddWorker element : this.worker) {
+            try {
+                element.join();
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+        }
         this.solr.close();
     }