add durable sse subscriptions

[jkarneges]

This enables SSE clients to establish durable subscriptions, by including query parameter durable=true. When durability is enabled, the any messages retained in KV Store are replayed upon establishment.

The IDs used in SSE events are a concatenation of the topics of interest and their last known retained versions, e.g.:

event: message
id: jtest:d06fe8f6d007c421-8,jtest2:a7b89db793b3da37-5
data: other msg 1

event: message
id: jtest:d06fe8f6d007c421-8,jtest2:a7b89db793b3da37-6
data: other msg 2

If a client reconnects and specifies a received event ID in the Last-Event-ID request header, the server can avoid replaying messages the client has already seen.

Messages are delivered by publishing hint actions to Fanout which trigger message fetching. This avoids the need to personalize the event IDs in published payloads. Later on, we can look at implementing such personalization to avoid fetching.

[deg4uss3r]

I wonder if we should add some safeguards around this indexing and throw a bad-request error if it fails?

[deg4uss3r]

This might be my misunderstanding but if we hit this case here should we return a 200?

[deg4uss3r]

I think this is just .map(|s| s)

else {
     req.get_header_str("Last-Event-ID").map(|s| s)
};