Use an existing library for a worker pool

[semanser]

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We should use some solution as the worker pool, instead of implementing our own (which is blocking on every operation).

https://github.com/semanser/codel/blob/b10a9daf6a436a3d1f9e3dba783a81801d51b959/backend/executor/queue.go#L26-L32

So far the closest option to what we need is pond - https://github.com/alitto/pond.