9535: internal: remove proc macro management thread r=jonas-schievink a=jonas-schievink
Communication with the proc macro server process has always happened one request at a time, so the additional thread isn't really needed (it just forwarded each request, and sent back the response). This removes some indirection that was a bit hard to understand (a channel was allocated and sent over another channel to return the response).
Hope I'm not missing anything here
Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>
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