configmaplister, reduce trigger status churn

[vaikas]

Addresses #3591

As part of debugging the #3591 there were couple of things we could do better to reduce the churn on the API server, namely use configmap lister instead of hitting the API server. Also use the same logic for deciding whether to update Trigger Status just like we would do if we were using genreconciler.

Proposed Changes

• use configmap lister
• reduce trigger status churn

Release Note

Docs

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vaikas

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• OWNERS [vaikas]

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[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/mtbroker/broker.go	85.4%	86.3%	0.9
pkg/reconciler/mtbroker/controller.go	84.6%	85.2%	0.6
pkg/reconciler/mtbroker/trigger.go	87.7%	87.9%	0.1

[lionelvillard]

/lgtm