Improve SSE implementation

[kabir]

A review on #820 showed that our SSE implementation might be too naive:

The current SSE implementation incorrectly dispatches each line starting with data: as a separate message. According to the SSE specification, multiple data: lines should be buffered and concatenated (with newlines) until an empty line is encountered, at which point the complete event should be dispatched. This will cause JSON parsing failures for any multi-line payloads.

[jmesnil]

FWIW https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#event_stream_format to correctly implement the SSE format.

It could be interesting to also add event to our stream (eg: event: <message|task| statusUpdate|artifactUpdate)

[ehsavoie]

We could try to reuse the implementation I wrote for vertx

[ehsavoie]

@jmesnil does that means that we wouldn't rely on the data events only anymore ?

[jmesnil]

@ehsavoie no, the idea was to identify the data with an event something like:

event: statusUpdate
data: {"statusUpdate": {"taskId": "task-uuid", "status": {"state": "TASK_STATE_COMPLETED"}}}

but this is silly, the type of events is already clear from the data.

[ehsavoie]

Well it could 'help' if you want to subscribe to only a single source/type of event but then you would need to register to every type