code: func (c *RegionCache) insertRegionToCache(r *Region) {
  	old := c.mu.sorted.ReplaceOrInsert(newBtreeItem(r))
  	if old != nil {
  		delete(c.mu.regions, old.(*btreeItem).region.VerID())
  	}
  	c.mu.regions[r.VerID()] = &CachedRegion{
  		region:     r,
  		lastAccess: time.Now().Unix(),
  	}
  }
docs: // insertRegionToCache tries to insert the Region to cache.