schedulers/balance_leader: could we use binarysearch instead of storeIndexMap in candidateStores

[AndreMouche]

Enhancement Task

schedulers/balance_leader: could we use binarysearch instead of store the index in storeIndexMap for candidateStores since the stores is already sorted:

pd/server/schedulers/balance_leader.go

Lines 250 to 255 in c8775b6

	type candidateStores struct {
	stores []*core.StoreInfo
	storeIndexMap map[uint64]int
	index int
	compareOption func([]*core.StoreInfo) func(int, int) bool
	}