search: New old search implementation (#39956)

This is an in-progress work on changing how task scheduler affects
performance of project search. Instead of relying on tasks being
executed at a discretion of the task scheduler, we want to experiment
with having a set of "agents" that prioritize driving in-progress
project search matches to completion over pushing the whole thing to
completion. This should hopefully significantly improve throughput &
latency of project search.

Release Notes:

- Improved project search performance

---------

Co-authored-by: Smit Barmase <smit@zed.dev>
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

crates/remote_server/src/headless_project.rs

@@ -639,9 +639,15 @@ impl HeadlessProject {
             PathStyle::local(),
         )?;
         let results = this.update(&mut cx, |this, cx| {
-            this.buffer_store.update(cx, |buffer_store, cx| {
-                buffer_store.find_search_candidates(&query, message.limit as _, this.fs.clone(), cx)
-            })
+            project::Search::local(
+                this.fs.clone(),
+                this.buffer_store.clone(),
+                this.worktree_store.clone(),
+                message.limit as _,
+                cx,
+            )
+            .into_handle(query, cx)
+            .matching_buffers(cx)
         })?;
 
         let mut response = proto::FindSearchCandidatesResponse {