do not create a new ObjectMapper per entry

also read value with MappingIterator.
This made reading 20-30 times faster. 
We can now read and index 100k-500k per second.
The varianz might be due to LuDB slowness.

performanceDb/src/main/java/org/lucares/performance/db/PerformanceDb.java

@@ -91,7 +91,7 @@ public class PerformanceDb implements AutoCloseable {
 				writer.write(entry);
 				count++;
 
-				if (count == 10000) {
+				if (count == 100000) {
 					final long end = System.nanoTime();
 					final double duration = (end - start) / 1_000_000.0;
 					LOGGER.info("inserting the last " + count + " took " + duration + " ms; " + durationInManager

performanceDb/src/main/java/org/lucares/performance/db/TagsToFile.java

@@ -65,6 +65,7 @@ public class TagsToFile implements AutoCloseable, CollectionUtils {
 			}
 		} catch (final NullPointerException e) {
 			// TODO @ahr ludb should handle unknown fields better
+			e.printStackTrace();
 		}
 
 		Collections.sort(result, PdbFileByTimeAsc.INSTANCE);