[base-controller] ControllerMessenger should prohibit retroactive subscriptions to published events

[MajorLift]

Motivation

Listeners in typical event systems like EventEmitter only trigger if they existed when the event was emitted. However, the messenger event pubsub system in our ControllerMessenger class does not follow this norm. This behavior should be fixed for maintenance and predictability reasons.

Acceptance Criteria

• Subscribers should receive events only if they were already registered to the event at the time it was published.
• After a given event has been published, its subscribers collection should not be retroactively mutated.
  • Eliminate the risk of a mutable subscribers collection triggering an infinite loop in React/Redux due to a circular dependency (e.g. a subscription handler mutates a hook dependency, which re-triggers its subscription).

Further Research

• Consider introducing "Topics" for a collection of events that may need to be published repeatedly, potentially to a growing list of subscribers.