nsqd: ephemeral topics

We have a few cases where we would like to stream close to realtime actionable data. The data in this case is less useful once it's in the past and not going to get stored/saved. So if a channel is not connected or configured on a MyCoolTopic#ephemeral messages are just discarded until a channel exist (ephemeral or not).

Currently we have to run a channel to /dev/null so the  ephemeral channel doesn't  burst with old data if the client was offline for a bit. 

Thoughts?