r-a now has 2 different types of diagnostics: 
* "disk-based" ones that come from `cargo check` and related, that emit
`project::Event::DiskBasedDiagnosticsStarted` and
`DiskBasedDiagnosticsFinished`
* "flycheck" diagnostics from r-a itself, that it tries to dynamically
apply to every buffer open, that come with `DiagnosticsUpdated` event.

Latter diagnostics update frequently, on every file close and open, but
`diagnostics.rs` logic had never polled for new diagnostics after
registering the `DiagnosticsUpdated` event, so the only way we could
have newer diagnostics was to re-open the whole panel.
The PR fixes that, and also adds more debug logging to the module.
The logic of the fix looks very familiar to previous related fix:
https://github.com/zed-industries/zed/pull/3128

One notable thing after the fix: "flycheck" diagnostics stay forever if
the diagnostics panel is opened: excerpts in that panel do not allow the
buffer to get dropped (hence, closed in terms of r-a) and get the
updated, zero diagnostics.
If the diagnostics panel is opened and closed multiple times, those
errors gradually disappear.

Release Notes:

- Fixed diagnostics panel not refreshing its contents properly