Change backingchannel to return a combinedchannelable

[lberk]

Fixes #3051

/wip

[lberk]

/hold
still a WIP

[matzew]

/woof

[knative-prow-robot]

@matzew:

In response to this:

/woof

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[lberk]

/unhold

[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/channel/channel.go	75.5%	77.8%	2.3

[vaikas]

/lgtm
/approve
Thanks!!!

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lberk, vaikas

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[knative-prow-robot]

@lberk: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-knative-eventing-build-tests	08c109e	link	/test pull-knative-eventing-build-tests

Full PR test history. Your PR dashboard.

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