Throttle TiKV's total region count in allocate IDs

## Development Task
Today it's necessary to limit the region count per TiKV, such as memory consumption.
In either raftstore v1 or v2 (partitioned-raft-kv), one tikv has a specific limit of region counts.
We probably want to fail the allocate ID when the total region count per TiKV exceeds a specific threshold. For v1, it can be 100K per 32GB, For v2, it can be 5K per 32GB.