This PR changes how we persist threads in Assistant2 to use `serde_json`
instead of `bincode` for the representation.

This makes the format more flexible to work with (and will allow for
using things like `#[serde(default)]`) if the schema changes over time.

Note: We have to bump the LMDB database version for this, so any threads
created before now will be gone.

Release Notes:

- N/A