managed_db.go
func (db *DB) SetDiscardTs(ts uint64) {
	if !db.opt.managedTxns {
		panic("Cannot use SetDiscardTs with managedDB=false.")
	}
	db.orc.setDiscardTs(ts)
}
func (db *DB) SetDiscardTs(ts uint64) {
	if !db.opt.managedTxns {
		panic("Cannot use SetDiscardTs with managedDB=false.")
	}
	db.orc.setDiscardTs(ts)
}
// SetDiscardTs sets a timestamp at or below which, any invalid or deleted
// versions can be discarded from the LSM tree, and thence from the value log to
// reclaim disk space. Can only be used with managed transactions.