FlushAccounting::StatisticsUpdate::update incorrect assumption about collection modify

Spotted this code which is now incorrect, the comment certainly is incorrect.

void FlushAccounting::StatisticsUpdate::update(ssize_t diskSizeDelta) {

    // System events don't get updated so just set flushedItem to true

    flushedItem = true;

    updateDiskSize(diskSizeDelta);

}

Since adding ModifyCollection (first added for CDC support) this is untrue, modify generates a new event and each modify updates that event. TTL modifcation was added in 7.6 and again can update system events.

The flushedItem thing there relates to avoiding the triggering of compaction if an empty collection is dropped, so needs some checking over and fixing. Flush work will also affect this.

For now aiming for morpheus unless some real badness is detected