Support last-event-id for SSE connections by buffering events

[kevinbader]

In order to enhance the reliability of event transmission, the SSE endpoint should support the last-event-id field as sent by the client. The idea is to maintain a buffer of events for each connection, similar to how events are buffered for the longpolling endpoint.

The size of the buffer should be configurable (likely related to #236).

[kevinbader]

@Azer0s does #279 implement this?

[Azer0s]

@kevinbader As of right now, #279 supports "last_event_id" in the query params. Do mean that by "field"?