HotEntryCache will update Instants only once per second

Calling Instant.now() several hundred thousand times per
second can be expensive. In my measurements >10% of the
time spend when loading new data was spend calling
Instant.now().
Fixed this by storing an Instant as static member and
updating it periodically in a separate thread.

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

@@ -152,7 +152,8 @@ public class DataStore implements AutoCloseable {
 
 	// A Doc will never be changed once it is created. Therefore we can cache them
 	// easily.
-	private final HotEntryCache<Long, Doc> docIdToDocCache = new HotEntryCache<>(Duration.ofMinutes(10));
+	private final HotEntryCache<Long, Doc> docIdToDocCache = new HotEntryCache<>(Duration.ofMinutes(10),
+			"docIdToDocCache");
 
 	private final DiskStorage diskStorage;
 	private final Path diskStorageFilePath;