simplify caching in TagsToFile

- PdbFiles no longer require dates to be monotonically
  increasing. Therefore TagsToFile does not have to ensure
  this. => We only have one file per Tags.
- Use EhCache instead of HashMap.

pdb-ui/src/test/java/org/lucares/performance/db/ingestor/TcpIngestorTest.java

@@ -84,17 +84,17 @@ public class TcpIngestorTest {
 
 	@Test
 	public void testIngestionThreadDoesNotDieOnErrors() throws Exception {
-		final OffsetDateTime invalidDate = OffsetDateTime.ofInstant(Instant.ofEpochMilli(-1), ZoneOffset.UTC);
+		final OffsetDateTime dateA = OffsetDateTime.ofInstant(Instant.ofEpochMilli(-1), ZoneOffset.UTC);
 		final OffsetDateTime dateB = OffsetDateTime.now();
 		final String host = "someHost";
 
 		try (TcpIngestor tcpIngestor = new TcpIngestor(dataDirectory)) {
 			tcpIngestor.start();
 
-			// skipped, because the date is invalid
+			// has a negative epoch time milli and negative value
 			final Map<String, Object> entryA = new HashMap<>();
-			entryA.put("duration", 1);
-			entryA.put("@timestamp", invalidDate.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
+			entryA.put("duration", -1);
+			entryA.put("@timestamp", dateA.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
 			entryA.put("host", host);
 			entryA.put("tags", Collections.emptyList());
 
@@ -109,18 +109,25 @@ public class TcpIngestorTest {
 			entryB.put("tags", Collections.emptyList());
 
 			final ObjectMapper objectMapper = new ObjectMapper();
-			final String data = objectMapper.writeValueAsString(entryA) + "\n" + corrupEntry + "\n"
-					+ objectMapper.writeValueAsString(entryB) + "\n";
+			final String data = String.join("\n", //
+					objectMapper.writeValueAsString(entryA), //
+					corrupEntry, //
+					objectMapper.writeValueAsString(entryB)//
+			)//
+					+ "\n";
 
 			PdbTestUtil.send(data);
 		}
 
 		try (PerformanceDb db = new PerformanceDb(dataDirectory)) {
 			final LongList result = db.get("host=" + host).singleGroup().flatMap();
-			Assert.assertEquals(result.size(), 2);
+			Assert.assertEquals(result.size(), 4);
 
-			Assert.assertEquals(result.get(0), dateB.toInstant().truncatedTo(ChronoUnit.MILLIS).toEpochMilli());
-			Assert.assertEquals(result.get(1), 2);
+			Assert.assertEquals(result.get(0), dateA.toInstant().truncatedTo(ChronoUnit.MILLIS).toEpochMilli());
+			Assert.assertEquals(result.get(1), -1);
+
+			Assert.assertEquals(result.get(2), dateB.toInstant().truncatedTo(ChronoUnit.MILLIS).toEpochMilli());
+			Assert.assertEquals(result.get(3), 2);
 		}
 	}
 }