func parseObjectID(id string) *bson.ObjectId {
	if id != "" && bson.IsObjectIdHex(id) {
		oid := bson.ObjectIdHex(id)
		return &oid
	}
	return nil
}
parseObjectID returns a bson.ObjectId from an hex representation of an object id or nil
if an empty string is passed or if the format of the id wasn't valid
package oplog
import (
	"errors"
	"strconv"
	"time"
	"gopkg.in/mgo.v2/bson"
)
type LastID interface {
	String() string
	Time() time.Time
}
type OperationLastID struct {
	*bson.ObjectId
}
type ReplicationLastID struct {
	int64
	fallbackMode bool
}