Find a better place for the pos persistance

The `pos` is a token which keeps track of where the client got in the sync stream for sliding sync. It's similar to the `next-batch` token concept from sync v2.

As such, it is useful to persist the `pos` between client restarts so the client may continue where they left off.

Since we have relatively complex multi-process situation on iOS, this value had to be stored in a cross-process safe manner.

As such we introduced a hack to persist the `pos` in the crypto store.

https://github.com/matrix-org/matrix-rust-sdk/blob/d039dad23af9fc7bd2181b668c96e68be7594532/crates/matrix-sdk/src/sliding_sync/cache.rs#L56-L69

This works relatively well, but users which don't use the end-to-end encryption feature lose this functionality. We should find a new home for this value.

Since the event cache store is now cross-process compatible it seems like the best candidate.

Start with crates/matrix-sdk/src/sliding_sync/cache.rs around the linked persistence code, then inspect the event cache store's cross-process storage APIs. Trace how the sliding-sync pos is read and written, move its persistence to the event cache store, and verify that clients without end-to-end encryption retain the position across restarts.

Written by the indexing model from the issue text.