Fix/mitigate data race in kafka source

[slinkydeveloper]

Signed-off-by: Francesco Guardiani francescoguard@gmail.com

(Maybe) mitigate/fixes #1262

From the diagnostic (#1262 (comment)), for some reason looks like Cleanup() is invoked two times, but i can't find why (I've analysed the code in sarama but it doesn't look like their problem). To mitigate it, I've added a sync.Once.

I've also moved the code to merge the error channels in another function

[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/common/pkg/kafka/consumer_factory.go	100.0%	83.9%	-16.1
kafka/common/pkg/kafka/consumer_handler.go	88.9%	89.5%	0.6

[n3wscott]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[slinkydeveloper]

/retest