Region sequence id may go backward after split or merge

The problem is that, now we have markers which will be written to WAL but not in store file. For a normal region close, we will write a sequence id file under the region directory, and when opening we will use this as the open sequence id. But for split and merge, we do not copy the sequence id file to the newly generated regions so the sequence id may go backwards since when closing the region we will write flush marker and close marker into WAL...