11281: ide: parallel prime caches r=jonas-schievink a=jhgg
cache priming goes brrrr... the successor to #10149
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this PR implements a parallel cache priming strategy that uses a topological work queue to feed a pool of worker threads the crates to index in parallel.
## todo
- [x] should we keep the old prime caches?
- [x] we should use num_cpus to detect how many cpus to use to prime caches. should we also expose a config for # of worker CPU threads to use?
- [x] something is wonky with cancellation, need to figure it out before this can merge.
Co-authored-by: Jake Heinz <jh@discordapp.com>
2cb85c14b6