Before this change we'd spawn N tasks in parallel on every keystroke,
afterwards
we only allow 1 background diagnostic refresh in flight at a time.

This also fixed a bug where we'd send O(n*2) pull diagnostic requests
when
re-opening a workspace with n editors.

Co-authored-by: John Tur <john-tur@outlook.com>

Closes #ISSUE

Release Notes:

- Improved performance when a large number of files were open by making
background diagnostics more efficient

---------

Co-authored-by: John Tur <john-tur@outlook.com>