Initial syncs being cached means you can get an outdated view of account data

[reivilibre]

There are two problems with this:

• (1) it possibly betrays client expectations, that doing an initial sync will get them to 'correct' state immediately.
• (2) it means we don't apply the correct set of ignored users to the events received in the timeline of the initial sync. (For context: the server hides events sent by ignored users, when sending events to the client.)

This issue is causing element-hq/element-ios#7387 — we could tell them to just do a zero-timeout incremental sync immediately but that feels like a hack and it wouldn't solve (2).

For (2), I wonder if we can just fetch the latest account data stream ID for the ignored users list, when doing an initial sync, and embedding it into the sync response cache token.
That way, the sync response cache entry will automatically be invalidated when the account data changes.

It's not a perfect solution since it only helps with ignored users, but it at least solves (2).
Problem (1) might be solvable through better documentation about the fact that clients should always incrementally sync after an initial sync if they want to guarantee they have the freshest data. Sliding sync might obviate these concerns anyway.