[ytap] Use a ymsg service to tap into a running ETL

[preaction]

It would be really nice if I could tap into a running ETL to see what it's doing. Since ymsg (#46) will allow us to send/recv messages, it'd be nice if there was a way to send messages on-demand without interrupting the workflow.

Normally, we could just use ymsg in a send mode, and then any number of listeners can listen for messages. But what if we don't want to make the job distributed?

This might make more sense as part of ymsg...

[preaction]

This is different from #141, because this involves the data. ytap spout or ymsg spout would create a tapping point so that anyone could plug in to the data stream. Now I'm thinking this should be part of ymsg...

[preaction]

ymsg tee is another possible word. The endpoint would define how it performs: if it connects out to somewhere else, or if someone connects in to it.