feat(server/v2): add EventSource to get_handler

add the query parameters "stream" and "streamIndex" to create
an EventSource (http://www.w3.org/TR/eventsource/) for processing
multiple updates without having to make multiple "wait" requests

the "streamIndex" parameter indicates where to start in the same way
as "waitIndex" on wait requests. it can also be overriden with the
EventSource standard "Last-Event-ID" header