Learner peers cannot be scheduled using placement settings

[lcwangchao]

When we are creating a table with learner placement settings like:

CREATE TABLE t(a int) LEARNERS=2;

The schedule will hang when scheduling learner peers. It is caused by tikv bug: tikv/tikv#11463 .

Temporary one way to walk around is to make a two phase operation:

1. Add rules to PD without learner rules and waiting the schedule done.
2. Add learner rules then.

[lcwangchao]

fixed by tikv/pd#4534

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.