Add TTL to the Broker.

[Harwayne]

Fixes #900

Proposed Changes

• A given Event can only go through a Trigger's reply back into the Broker 10 times in a row. On the 11th iteration, the event will be dropped (accepted by the Ingress, but ignored).
  • Note that this only affects Trigger replies, not any requests made directly by the Trigger's Subscriber.
  • This is to prevent accidental infinite loops where a function replies with the event it consumed.
  • 10 is a hard coded default, but later we can make this part of the Broker's spec to allow customization.
• Nothing outside of the Broker/Trigger infrastructure will ever see the TTL.

Release Note

An Event can flow through a Broker via Trigger replies only 10 times before getting dropped.

[vaikas]

/lgtm
/approve

[Harwayne]

/test pull-knative-eventing-integration-tests

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/broker/receiver.go	82.9%	84.8%	1.9
pkg/broker/ttl.go	Do not exist	100.0%

[Harwayne]

/test pull-knative-eventing-integration-tests

[vaikas]

/test pull-knative-eventing-integration-tests

[vaikas]

/lgtm
/approve

[knative-prow-robot]

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