txn.go
func (o *oracle) setDiscardTs(ts uint64) {
	o.Lock()
	defer o.Unlock()
	o.discardTs = ts
}
func (o *oracle) setDiscardTs(ts uint64) {
	o.Lock()
	defer o.Unlock()
	o.discardTs = ts
}
// Any deleted or invalid versions at or below ts would be discarded during
// compaction to reclaim disk space in LSM tree and thence value log.