Use transformer to avoid making unnecessary event copy in delivery transformer

[ian-mi]

When sending to a retry topic it is still necessary to clone EventContext since the retry client
does not yet use message bindings.

Fixes #1511

Benchmark results show significant improvement for large payload sizes since this avoids buffering payload data:

DeliveryNoReply/0_bytes-16 9.68µs ± 1% 9.26µs ± 2% -4.28% (p=0.000 n=9+10)
DeliveryNoReply/1000_bytes-16 10.4µs ± 2% 9.5µs ± 1% -8.42% (p=0.000 n=9+10)
DeliveryNoReply/1000000_bytes-16 294µs ± 2% 146µs ± 0% -50.28% (p=0.000 n=10+9)
DeliveryNoReplyFakeClient/0_bytes-16 2.63µs ± 1% 2.45µs ± 1% -6.70% (p=0.000 n=8+10)
DeliveryNoReplyFakeClient/1000_bytes-16 2.81µs ± 3% 2.47µs ± 1% -12.14% (p=0.000 n=10+10)
DeliveryNoReplyFakeClient/100000_bytes-16 14.9µs ± 1% 2.5µs ± 2% -83.51% (p=0.000 n=10+9)

[knative-prow-robot]

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[yolocs]

Thanks for picking this up so quick. Left a nit.

[ian-mi]

Event messages are not immutable so I had to introduce an immutable version to avoid the event copy. This happened to expose a bug in the HTTP message implementation which I fixed upstream in cloudevents/sdk-go#557.

[ian-mi]

/retest

[yolocs]

/lgtm

Left a question

[ian-mi]

/retest

[yolocs]

/lgtm

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/broker/eventutil/hops.go	85.0%	84.2%	-0.8
pkg/broker/eventutil/immutable_event_message.go	Do not exist	100.0%
pkg/broker/handler/processors/deliver/processor.go	79.5%	80.0%	0.5

[yolocs]

/retest

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-google-knative-gcp-integration-tests	2020-07-27 23:52:44.559 +0000 UTC	1/3
pull-google-knative-gcp-wi-tests	2020-07-28 16:12:44.596 +0000 UTC 2020-07-28 16:52:14.397 +0000 UTC	2/3

Automatically retrying due to test flakiness...
/test pull-google-knative-gcp-wi-tests