replicator: fixed infinity loop on sync replication

Before this fix, replicator adds the same lookup into callbacks over and
over, until it reaches out of memory, and crashes.

This regression was introduced 447e086
with initial implementation of replicator.

src/replication/replicator/replicator-queue.c

@@ -333,6 +333,7 @@ replicator_queue_handle_sync_lookups(struct replicator_queue *queue,
 		} else {
 			array_push_back(&callbacks, &lookups[i]);
 			array_delete(&queue->sync_lookups, i, 1);
+			count--;
 		}
 	}