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.

pdb-utils/src/test/java/org/lucares/utils/cache/HotEntryCacheTest.java

@@ -45,6 +45,7 @@ public class HotEntryCacheTest {
 		cache.put("key", "value1");
 
 		clock.plusSeconds(2);
+		cache.updateTime();
 
 		cache.put("key", "value2");
 
@@ -64,19 +65,23 @@ public class HotEntryCacheTest {
 		Assert.assertEquals(cachedValue1_evicted, null);
 	}
 
-	// TODO that does not make sense. Get should not evict. We should be happy that
-	// the element is still in the map when we need it.
-	public void testGetEvicts() throws Exception {
+	public void testGetTouches() throws Exception {
 		final ModifiableFixedTimeClock clock = new ModifiableFixedTimeClock();
 		final Duration timeToLive = Duration.ofSeconds(10);
 		final HotEntryCache<String, String> cache = new HotEntryCache<>(timeToLive, clock);
 
 		cache.put("key", "value1");
 
+		// skip forward in time, but do not yet trigger eviction
 		clock.plus(timeToLive.plusMillis(1));
+		cache.updateTime();
 
-		final String cachedValue1_evicted = cache.get("key");
-		Assert.assertEquals(cachedValue1_evicted, null);
+		cache.get("key"); // will touch the entry
+
+		cache.triggerEvictionAndWait(); // if get didn't touch, then this will evict the entry
+
+		final String cachedValue1 = cache.get("key");
+		Assert.assertEquals(cachedValue1, "value1");
 	}
 
 	public void testEvictionByBackgroundThread() throws InterruptedException, ExecutionException, TimeoutException {
@@ -92,6 +97,7 @@ public class HotEntryCacheTest {
 		cache.put("key", "value1");
 
 		clock.plus(timeToLive.minusSeconds(1));
+		cache.updateTime();
 
 		cache.put("key2", "value2");
 		clock.plus(Duration.ofSeconds(1).plusMillis(1));
@@ -149,6 +155,7 @@ public class HotEntryCacheTest {
 
 		// seek, so that it is almost evicted
 		clock.plus(timeToLive.minusMillis(1));
+		cache.updateTime();
 
 		// the for each should touch the entries
 		cache.forEach(s -> {
@@ -156,12 +163,14 @@ public class HotEntryCacheTest {
 
 		// seek again
 		clock.plus(timeToLive.minusMillis(1));
+		cache.triggerEvictionAndWait();
 
 		// if the touch didn't happen, then the value is now evicted
 		Assert.assertEquals(evictionEventFuture.isDone(), false);
 
 		// seek again, so that the entry will get evicted
 		clock.plus(timeToLive.minusMillis(1));
+		cache.triggerEvictionAndWait();
 
 		Assert.assertEquals(cache.get("key"), null);
 	}