code: func LeastConnPolicy() GroupPolicyHandler {
  	return func(g *EngineGroup) *Engine {
  		var slaves = g.Slaves()
  		connections := 0
  		idx := 0
  		for i := 0; i < len(slaves); i++ {
  			openConnections := slaves[i].DB().Stats().OpenConnections
  			if i == 0 {
  				connections = openConnections
  				idx = i
  			} else if openConnections <= connections {
  				connections = openConnections
  				idx = i
  			}
  		}
  		return slaves[idx]
  	}
  }
docs: // LeastConnPolicy implements GroupPolicy, every time will get the least connections slave