receive adapters can now run in leader elected mode

[lionelvillard]

For #3157

Proposed Changes

• Receive adapter can now run in leader elected mode

Release Note

Docs

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lionelvillard

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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/adapter/v2/config.go	70.8%	64.5%	-6.3
pkg/adapter/v2/main.go	61.0%	63.3%	2.3
pkg/adapter/v2/main_message.go	55.3%	58.7%	3.4

[cr22rc]

/lgtm

[knative-prow-robot]

@cr22rc: changing LGTM is restricted to collaborators

In response to this:

/lgtm

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[aslom]

/lgtm