Trigger responses go back into the Broker

[Harwayne]

#788 creates the Broker and Trigger objects, but any response from a Trigger is simply dropped. It should be sent back into the Broker.

[Harwayne]

Harwayne#24 and Harwayne#25 are two ways to do this. The latter is preferred, as we expect to move to the CloudEvents SDK anyway (see #891).

[akashrv]

/kind bug
related to #905