Support discarding events

[asnare]

Currently an incoming event is always mapped to an Avro record and passed on to the sink. There are situations, however, where it can make sense to discard the event instead of completing the mapping and forwarding a record to the sinks.

Two trivial use-cases which spring to mind include:

• Whitelisting domains for which events should be collected.
• Suppressing legacy event types.

In addition, once pull request #115 is closed this could serve as a workaround in lieu of issue #6.