Follow-up of https://github.com/zed-industries/zed/pull/3225
That PR enabled every `project::Event::DiskBasedDiagnosticsFinished` to
update the diagnostics, which turned out to be bad, Zed does query for
more diagnostics after every excerpt update, and that seems to be due to
`Event::Edited` emitted by the multibuffers created in the diagnostics
panel.

* now, instead of eagerly updating the diagnostics every time, only do
that if the panel has 0 or 1 caret placed and no changes were made in
the panel yet.
Otherwise, use previous approach and register the updated paths to defer
their update later.

* on every `update_excerpts` in the diagnostics panel, query the entire
diagnostics summary (and store it for the future comparisons), compare
old and new summaries and re-query diagnostics for every path that's not
in both summaries.
Also, query every path that was registered during the
`DiskBasedDiagnosticsFinished` updates that were not eagerly updated
before.

This way we're supposed to get all new diagnostics (for new paths added)
and re-check all old paths that might have stale diagnostics now.

* do diagnostics rechecks concurrently for every path now, speeding the
overall process

Release Notes:

- Fixed diagnostics triggering too eagerly during multicaret edits and
certain stale diagnostics not being removed in time