Add debug log that shows up in trace whenever CacheLoader#load is invoked

This allows us to see cache misses in the trace.

Change-Id: I7ed1c779c35e6621cabaa6f257289913c92e1fe9
Signed-off-by: Edwin Kempin <ekempin@google.com>

java/com/google/gerrit/server/cache/h2/H2CacheImpl.java

@@ -235,6 +235,8 @@ public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> implements Per
 
     @Override
     public ValueHolder<V> load(K key) throws Exception {
+      logger.atFine().log("Loading value for %s from cache", key);
+
       if (store.mightContain(key)) {
         ValueHolder<V> h = store.getIfPresent(key);
         if (h != null) {