Do not allow concurrent FindAllReferences requests for the same multibuffer anchors (#9242)

FindAllReferences LSP requests might take a long time to complete, and
currently Zed allows multiple requests spawned concurrently for the same
Anchor in the multi buffer. That results in multiple search results'
multi buffers appearing, sometimes at once, which is not what we want.

Part of https://github.com/zed-industries/zed/issues/5351 that helps to
reduce the amount of search results after clicks that did not resolve
instantly.


Release Notes:

- Improved FindAllReferences action by not allowing concurrent requests
for the same multi buffer source