* feat: Update WAL plugin for new structure
This ended up being a very large change set. In order to get around circular dependencies, the processing engine had to be moved into its own crate, which I think is ultimately much cleaner.
Unfortunately, this required changing a ton of things. There's more testing and things to add on to this, but I think it's important to get this through and build on it.
Importantly, the processing engine no longer resides inside the write buffer. Instead, it is attached to the HTTP server. It is now able to take a query executor, write buffer, and WAL so that the full range of functionality of the server can be exposed to the plugin API.
There are a bunch of system-py feature flags littered everywhere, which I'm hoping we can remove soon.
* refactor: PR feedback
7230148b58