Topology cleanup (#325)

* Use types to enforce topology state machine

* Kill off receive_lines (in favor of receive) to make tests resiliant against healthchecks running

* Treat initial load of config as a reload from an empty config

* Kill off count_lines to make benches resiliant against healthchecks running

* Unify error handling around starting and reloading topology

* Add missing span for source tasks

* Remove redundant log