Encapsulate connections to external services (e.g: carbon) in a data structure

[rkallos]

Currently, the state regarding a connection to carbon is very tightly coupled to seer_server's state. This will limit our ability to reason about seer_server's state in the future, especially if we add the possibility of new metrics backends, such as Prometheus.

I suggest wrapping connections to external services in a data structure and exposing functions to inspect and manipulate that structure.