Implement the JACK API method jack_set_client_registration_callback

[dmansfield]

The JACK API method jack_set_client_registration_callback is used by a client to monitor connections and disconnections of other clients.

In turn, we will call a user supplied JS callback function with the client name, and a boolean (connected).

Within the JACK callback context it is unsafe to access V8 internals, so uv_async_send is used to notify a "safe" context where the callback is actually invoked. libuv may coalesce calls to the uv_async_cb so a linked list of information is maintained.