ConfigSetService: add CRUD operations, subsume ZkConfigManager

Increase the scope of ConfigSetService with CRUD operations and anything else so that it can be the one place to go for anything ConfigSet related. This should be easier to understand/maintain, and also improve the efficacy of the plug-ability of ConfigSetService.

It's likely ZkConfigManager will disappear, with most of its functionality moving into CloudConfigSetService (ZK). A number of places in SolrCloud use ZkConfigManager that will change to use the ConfigSetService instead.