This PR fixes an issue where the thread history would only work in one
Zed window at a time.
The backing LMDB database can only be opened once per Zed instance.
However, the `ThreadStore` has one instance per Zed window.
To fix this, we need to create the `heed` environment once and store it
as a global, and then reference the same environment across all of the
`ThreadStore`s.
Release Notes:
- N/A
Marshall Bowers
98ea659af6