Reduce log level for events that make most of the log volume with lea…

…st of newsworthiness (#22)

These messages make up most of the log volume, yet only indicate that
nothing has happened.

Let's reduce them to the `debug` log level.

src/Synchronizer.php

@@ -222,7 +222,7 @@ private function update(object $sourceObject, object $destinationObject): void
         }
 
         if (!$mapResult->getObjectHasChanged()) {
-            $this->logger->info('Kept object with id {id}.', ['id' => $this->mapper->idOf($sourceObject)]);
+            $this->logger->debug('Kept object with id {id}.', ['id' => $this->mapper->idOf($sourceObject)]);
 
             return;
         }