Reduce unavailable time caused by leader election

Currently, when a PD becomes leader, it needs to load all regions meta from storage. During the process, it cannot provide service such as TSO. There are some possible optimizations for this problem.

- Start service before loading regions.
Clients have region cache, sometimes they don't really need `GetRegion` to work. So start the TSO service earlier can help recover some transactions.

- Maintain region tree in follower's memory too.
Now with the `region-storage` feature, followers constantly sync regions with the leader. We can maintain the region tree in all followers, then it won't have to load from storage when becomes leader.