fix(sync): properly drop rpc subscriptions, add doc status

[Frando]

Description

So far in #1333, if a RPC or in-memory client called doc.subscribe() the event callback would never be dropped, even if the client did drop the event stream. This PR fixes this, by having the event callbacks return whether the callback should stay active or not. We can't use the removal token here, because calling LiveSync::unsubscribe from within the event callback would deadlock the actor.

Also adds a a LiveStatus to the doc info RPC call. For now only contains the number of subscribers. More info, e.g. on peers, can come later.

Notes & open questions

• As of feat: add iroh-sync and integrate into iroh node #1333 and unchanged by this PR: doc.subscribe will fail for documents that are not in the LiveSync (they are added via doc.import or doc.start_sync). This is unfortunate, because you'd often want to setup a subscription before starting sync, to catch all events.

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.