make CacheKey comparable

The CacheKey is used as a key in a HashMap. Lookup can
be faster if the CacheKey is comparable when there are
hash collisions.
In this case I was not able to measure any effect. I am
keeping the comparables nonetheless, because the can
only have a positive effect.

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

@@ -22,7 +22,7 @@ public class TagsToFile implements AutoCloseable {
 	private final static Logger METRICS_LOGGER_NEW_WRITER = LoggerFactory
 			.getLogger("org.lucares.metrics.ingestion.tagsToFile.newPdbWriter");
 
-	private static final class CacheKey {
+	private static final class CacheKey implements Comparable<CacheKey> {
 		private final Tags tags;
 
 		public CacheKey(final Tags tags) {
@@ -30,6 +30,11 @@ public class TagsToFile implements AutoCloseable {
 			this.tags = tags;
 		}
 
+		@Override
+		public int compareTo(final CacheKey o) {
+			return tags.compareTo(o.tags);
+		}
+
 		@Override
 		public int hashCode() {
 			final int prime = 31;
@@ -54,7 +59,6 @@ public class TagsToFile implements AutoCloseable {
 				return false;
 			return true;
 		}
-
 	}
 
 	private final static class RemovalListener implements EventListener<CacheKey, PdbWriter> {