Client-side improvements

[isc-bsaviano]

This PR addresses feedback from a DC beta tester.

• Limit concurrent file system reads on initial folder indexing to 250 to avoid EMFILE errors.
• Limit concurrent file update REST requests to 50 to avoid overloading the server.
• Always get the text of a document from the file system, even if VS Code has it loaded, to avoid concurrency issues when the file system updates before VS Code has updated its version.
• Avoid unneeded file system reads after export by updating the document index using the already known text of the file that was exported.
• New rate limiting utility code that is better documented

[gjsjohnmurray]

LGTM but I haven't tested it.

[isc-bsaviano]

Thanks John. I will give the issue reporter at least a few days to respond.