Deprecate Spider's high availability feature

Spider supports a high availability setting described in the following slides: https://www.slideshare.net/Kentoku/spider-ha-20100922dtt7

However, this feature is difficult to use properly, and it makes the Spider implementation quite complex. I (nayuta.yanagisawa) strongly recommend using other HA solutions for high availability and using Spider just for sharding or federation. So, we deprecate the feature.