Speed up tso service discovery

## Development Task
Issue Number: Ref https://github.com/tikv/pd/issues/5895

Currently, the pd client follows the path "discovers api leader->asks api leader for tso nodes->asks tso nodes in round-robin to find the primary of the keyspace group of the keyspace that the client belongs to", which is slower than before. The reason of the last part "asks tso nodes in round-robin ..." is that a tso node only knows the participants on the node and doesn't know the other participants. We need to let every tso node watch primaries of all keyspace group to get global picture, which will speed up both service discovery and getMembers API.