added synchronization to exists() call bacause the concurrent call to

that method showed in thread dump close to deadlock situations. Its also
better to synchronize IO operations because they become faster then.

source/net/yacy/cora/federate/solr/connector/EmbeddedSolrConnector.java

@@ -376,7 +376,7 @@ public class EmbeddedSolrConnector extends SolrServerConnector implements SolrCo
     }
 
     @Override
-    public boolean existsById(String id) {
+    public synchronized boolean existsById(String id) {
         return getCountByQuery("{!raw f=" + CollectionSchema.id.getSolrFieldName() + "}" + id) > 0;
     }