code: func NewRegionCache(pdClient pd.Client) *RegionCache {
  	c := &RegionCache{
  		pdClient: pdClient,
  	}
  	c.mu.regions = make(map[RegionVerID]*CachedRegion)
  	c.mu.sorted = btree.New(btreeDegree)
  	c.storeMu.stores = make(map[uint64]*Store)
  	return c
  }
docs: // NewRegionCache creates a RegionCache.