RFR: Precleaning should use GC workers to do actual work

[Roman Kennke]

Alright. Go for it then. The patch looks good.

> 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
>