Diagnostics updates were programmed in Zed based off the r-a LSP push
diagnostics, with all related updates happening per file.

https://github.com/zed-industries/zed/pull/19230 and especially
https://github.com/zed-industries/zed/pull/32269 brought in pull
diagnostics that could produce results for thousands files
simultaneously.

It was noted and addressed on the local side in
https://github.com/zed-industries/zed/pull/34022 but the remote side was
still not adjusted properly.

This PR 

* removes redundant diagnostics pull updates on remote clients, as
buffer diagnostics are updated via buffer sync operations separately
* batches all diagnostics-related updates and proto messages, so
multiple diagnostic summaries (per file) could be sent at once,
specifically, 1 (potentially large) diagnostics summary update instead
of N*10^3 small ones.

Buffer updates are still sent per buffer and not updated, as happening
separately and not offending the collab traffic that much.

Release Notes:

- Improved diagnostics performance in the collaborative mode