Store BKD blocks with continuous ids more efficiently

For scenes that index is sorted on the field, it could be a common situation that blocks have continuous ids. Maywe can handle this situation more efficiently (only write the first id of this block). And we can just check

stritylysorted && (docIds[start+count-1] - docids[start] + 1) == count

 to see if ids are continuous, the check should be very fast