libsql: refactor push and sync code

[LucioFranco]

This PR has multiple commits each broken up into their own refactor piece. Broken down is the list of refactors and why:

• Move sync ctx behind a mutex, this in general makes sense since we only want one push to be active at a time but also will enable the future durable_frame_no caching by allow us to mutate the struct on push. It is an async lock since we want to sync access to push over long async periods.
• move push_with_retry into sync.rs
• move push_one_frame into sync.rs these both make sense since the data they depend on is inside the sync_ctx struct so this allows us to make those fields private.
• Following up on those two, the next commit makes those fields private to improve code encapsulation.
• Small refactor now to use the auth_token field from the struct.

This PR just moves some code around but doesn't actually touch any of the logic in the code.

[LucioFranco]

Last commit contains a bit more since for some reason an old hyper commit got layered into the commit range.

[LucioFranco]

Should now not contain any hyper changes and should just be a refactor of the old functions.