Refactor agent check maps

The agent uses several different maps to keep track of the different checks. This complicates the lock handling and makes it easier to forget cases (see #3265). This should be refactored so that the check maps are replaced with a single data structure. This could be a `map[string]struct{...}` or a `map[string]interface{}` with type switching or a separate data structure altogether.

https://github.com/hashicorp/consul/blob/master/agent/agent.go#L109-L129