And waits for them. That's pointless.
{
public:
    MergeProgressCallback(
        MergeList::Entry & merge_entry_, UInt64 & watch_prev_elapsed_, MergeStageProgress & stage_)
        : merge_entry(merge_entry_)
        , watch_prev_elapsed(watch_prev_elapsed_)
        , stage(stage_)
    {
        updateWatch();
    }
    MergeList::Entry & merge_entry;
    UInt64 & watch_prev_elapsed;
    MergeStageProgress & stage;
    void updateWatch()