Add server->client send with cb + timeout support?

[ptaoussanis]

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:

• 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 #259.

[shader]

I think I'd like to have something like this, since I think clients should be able to subscribe to some events separately.

Here's the use case / pattern I'm considering:

• send "global events" to uid (notifications, chat messages, etc.)
• send "local events" to specific client, e.g. update events for the object/document that the client is viewing. Different clients from the same user will likely be viewing and thus subscribed to different things.

I think it should be somewhat straightforward to factor out the "client-id sending" part from the "uid sending" and then call it directly, given a client-id.