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/git/SearchingChangeCacheImpl.java

@@ -144,6 +144,7 @@ public class SearchingChangeCacheImpl implements GitReferenceUpdatedListener {
 
     @Override
     public List<CachedChange> load(Project.NameKey key) throws Exception {
+      logger.atFine().log("Loading changes of project %s", key);
       try (ManualRequestContext ctx = requestContext.open()) {
         List<ChangeData> cds =
             queryProvider