Current chsk-send! API:

• Clients: (fn [event & [?timeout-ms ?cb-fn]])
• Server: (fn [user-id event])

It may be useful to add a low-level (private?) server-side method like (fn [client-id event & [?timeout-ms ?cb-fn]]).

Uses include:

• Add util for bulk server->uids push? #421
• Could add a pong for ws-ping events issued by server, to mirror the new pong for ws-ping events issued by client to address How to detect loss of network connection client-side in Chrome #259.