Support the PD follower to provide the TSO service

## Development Task

The PD leader node undertakes a lot of work in the cluster, including some periodic operations that require high CPU and other resources, which is a potential factor affecting the quality of service for TSO services. In order to further avoid this impact, we can separate the TSO service provider from the PD leader, and let the PD follower provide the TSO service, so as to avoid the impact of other operations on the PD leader as much as possible, thereby providing a more stable Reliable TSO service.

- [ ] Support the PD follower to elect as a TSO leader to provide the service.
- [ ] Support the PD client to connect with the PD follower to send the TSO request.