Update event system to use a subscriber registry

[oubiwann]

Tasks:

• devise an appropriate mechanism for a global registry - going to use an in-memory db (see #13)
  • possibly a new :registry key on the event component with an associated data structure?
  • define an appropriate schema for the subscriber registry
  • needs to take into account topic(s), tag(s), and subscriber functions
  • identify the operations (functions) necessary to be called upon the registry
  • define the API and create the functions in the appropriate place
• migrate the current, draft subscriber functionality to the new subscriber registry + API
• utilize the newly-created API (above) for adding subscribers to the registry (this should be an action that is take upon event system initialization)
• testing: ensure that upon addition, subscription functions are ready to use/be applied

Depends upon #13 - "Add db component"