Add util for bulk server->uids push?

[ptaoussanis]

Currently the recommended way of broadcasting an event to multiple uids is something like this:

(doseq [uid all-uids] (chsk-send! uid [:my-app/ev {:payload some-data}]))

An inefficiency in this case is that the event message will be repeatedly serialized - once for each uid.

Instead, we could easily add a simple util that:

• Takes a coll of uids
• Uses the active packer only once
• Sends the packed string to all uids

This would be a substitute for [#276].