TiFlash replicate progress blocked when there is only 1 TiKV node

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

- Bootstrapped a cluster with 1 TiKV, 1 TiFlash, 1 PD and 1 TiDB.
- load some data into TiDB
- and then execute `alter table t set tiflash replica 1` to add the TiFlash replica for table t.

2. What did you expect to see?
- The TiFlash replica progress stays at 0%.
- After changed PD's `max-replicas` from 3 to 1, the progress still 0%.
- After execute `curl -X POST -d '{"group_id":"pd","id":"default","role":"voter","count":1}' http://127.0.0.1:2379/pd/api/v1/config/rule`, the replicate progress start to work.

3. What did you see instead?

- The TiFlash replicate progress works When there is only one TiKV node.
- After change `max-replicas` from 3 to 1, the replicate progress should work

4. What version of PD are you using (`pd-server -V`)?

Release Version: v4.0.0-beta.1-37-g0ba1160b
Git Commit Hash: 0ba1160bf0b5537ce62067643857ce5d191be659
Git Branch: release-4.0
UTC Build Time: 2020-03-11 01:54:01