schedulers/balance_leader: could we merge sourceCandidate and targetCandidate to one

## Enhancement Task
Could we merge `sourceCandidate` and `targetCandidate` into one, since they almost the same except the sequence of order is different: asc and desc. Meanwhile, they are interacted on each other when any store is changed.
[https://github.com/tikv/pd/blob/c8775b6176a03eb25cb1622fc46ba1d496872df7/server/schedulers/balance_leader.go#L325-L326](https://github.com/tikv/pd/blob/c8775b6176a03eb25cb1622fc46ba1d496872df7/server/schedulers/balance_leader.go#L325-L326)