This is comments for KeyValueScanner.getScannerOrder
* Get the order of this KeyValueScanner. This is only relevant for StoreFileScanners and
* MemStoreScanners (other scanners simply return 0). This is required for comparing multiple
* files to find out which one has the latest data. StoreFileScanners are ordered from 0
* (oldest) to newest in increasing order. MemStoreScanner gets LONG.max since it always
* contains freshest data.
As now we may have multiple memstore scanners, I think the right way to select scanner order for memstore scanner is to ordered from Long.MAX_VALUE in decreasing order.
But in CompactingMemStore and DefaultMemStore, the scanner order for memstore scanner is also start from 0, which will be messed up with StoreFileScanners.