trim docIdToDoc list

This reduces memory usage by 1 or 2 MB.
33% of an ArrayList can be free. If the list is 1 million entries long,
then the list wastes 2.6 MB.
The Doc objects in the list are much bigger.

data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java

@@ -70,6 +70,7 @@ public class DataStore {
 
 		});
 		trimIntLists();
+		((ArrayList<Doc>)docIdToDoc).trimToSize();
 	}
 
 	private void cacheTagToFileMapping(final Tags tags, final Path path) {