balance_region: remove unnecessary retry when pick region from a store.

## Enhancement Task
In scheduler balance region, we try and retry to pick a region from a store until success with the following priority:
- pending region
- follower
- leader
- learner

https://github.com/tikv/pd/blob/99e38eb7b4454cc283c0fea99518753a196b9b50/server/schedulers/balance_region.go#L170-L194
However, I think if we failed to pick a region from pending regions, the possibility of successfully pick a region from the pending region is very very low, so could we skip to pick from pending in the next retry once we get no region from pending regions?