Workspace symbol search can randomly take longer if files are being tag parsed

[sean-mcmanus]

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This is after the other performance fixes in 1.19.3. I've seen up to 5 seconds extra on Windows with a large workspace. Longer delays might be possible Linux and Mac or slower machines (not sure yet).

This might be fixable via preventing other threads from updating the database if a document or workspace symbol search is in progress.

[heartacker]

https://code.visualstudio.com/blogs/2018/03/23/text-buffer-reimplementation