RFR: Precleaning should use GC workers to do actual work

[Aleksey Shipilev]

On 10/03/2018 01:17 PM, Roman Kennke wrote:
> Is it possible and/or does it make sense to parallelize the precleaning?

Maybe? The bulk of the precleaning work is done during reference discovery, so precleaning is almost
never a time hog. We can try to do this upstream, but this is completely tangential to the problem
at hand.

-Aleksey