Implementation of Targets that knows it's backed by a configmap

[grantr]

Problem
Currently the Broker reconciler uses the in-memory variant of config.Targets to aggregate broker and trigger routing and handles persisting that to a configmap. This adds a lot of complexity to the reconciler: since reconciler threads can process multiple brokers at once, it has to run a separate goroutine to update the configmap safely. It would be nicer if there were a variant of Targets that wrapped up all the configmap load/store logic in the same interface.

It's also challenging to test this logic with normal reconciler table tests, because the configmap update happens outside the Reconcile method.

Persona:
Contributor

Exit Criteria
The Broker reconciler can delegate load/store to a separate library.

Additional context (optional)
This should probably wait until #815 is resolved.

[yolocs]

/assign

[grantr]

This is no longer necessary with #852 and #863. Can reopen if we decide it's still useful.