managed_db.go
func OpenManaged(opts Options) (*DB, error) {
	opts.managedTxns = true
	return Open(opts)
}
func OpenManaged(opts Options) (*DB, error) {
	opts.managedTxns = true
	return Open(opts)
}
// OpenManaged returns a new DB, which allows more control over setting
// transaction timestamps, aka managed mode.
//
// This is only useful for databases built on top of Badger (like Dgraph), and
// can be ignored by most users.