Issue dealing with concurrent requests

[tobie]

Root cause of whatwg/html#3726 seems to be that wattsi doesn't handle concurrent requests properly.

When pr-preview serializes the requests to the wattsi server, everything works fine.

If those requested are parallelized, the wattsi server returns partially empty zip files for one of the two requests.

This problem only surfaced recently.

[tobie]

Note that as pr-preview itself may handle multiple requests in parallel and that other services also rely on wattsi-server, it is very possible to have concurrent requests to wattsi server despite pr-preview now serializing requests within each pull request. That's an effective band aid, not a complete solution.