Do compression on build slaves

[ElvishJerricco]

I think it would be preferable for the Hydra server to run on a pretty lightweight instance, meaning it'd be better to do CPU intensive nar compression on build slaves. It also seems to do compression serially, so it'd also be good if multiple nars could be compressed simultaneously, potentially on different build slaves.

[RossComputerGuy]

+1 on this, I think it definitely would help. What if we had compression servers? Servers dedicated just for compression.

[mweinelt]

This will become possible as part of the new queue-runner. There will also be support for presigned urls, so the builders can take care of the upload to S3.

#1517
https://github.com/helsinki-systems/hydra-queue-runner