WIP: Refactor Linux platform implementation (#10227)

This puts the Linux platform implementation at a similar code style and
quality to the macOS platform. The largest change is that I collapsed
the `LinuxPlatform` -> `[Backend]` -> `[Backend]State` ->
`[Backend]StateInner` to just `[Backend]` and `[Backend]State`, and in
the process removed most of the `Rc`s and `RefCell`s.

TODO:
- [x] Make sure that this is on-par with the existing implementation
- [x] Review in detail, now that the large changes are done.
- [ ] Update the roadmap

Release Notes:

- N/A

crates/client/src/client.rs

@@ -784,7 +784,6 @@ impl Client {
             }
             Status::UpgradeRequired => return Err(EstablishConnectionError::UpgradeRequired)?,
         };
-
         if was_disconnected {
             self.set_status(Status::Authenticating, cx);
         } else {