Port KafkaChannel to new bindings API

[slinkydeveloper]

Fixes #976

[matzew]

cool, I will run some tests here

[slinkydeveloper]

/hold

I wish to fix the intermediate buffering that happens inside message receiver, which can be avoided for the kafka case

[slinkydeveloper]

Depends on knative/eventing#2773

[slinkydeveloper]

/unhold

[lberk]

/assign

[lberk]

Outside of those two nits this works for me
/lgtm
/hold

[slinkydeveloper]

/unhold

[lberk]

@slinkydeveloper did you test these changes? With the last commit the dispatcher doesn't forward any messages anymore for me.

Seems like the correct approach (for now) is to keep the variables assigned below the NewMessageReciever and remove the topicFunc assignment within the struct declaration. Please make sure to remove the duplicate assignement (as it wasn't removed in 9fc5704

[slinkydeveloper]

@slinkydeveloper did you test these changes? With the last commit the dispatcher doesn't forward any messages anymore for me.

Funny how the test pass...

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
kafka/channel/pkg/dispatcher/dispatcher.go	66.2%	61.1%	-5.2

[slinkydeveloper]

@lberk I've tested locally and it works :)

[lberk]

/lgtm
/approve

[n3wscott]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lberk, n3wscott, slinkydeveloper

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