Having a hundred threads doing I/O at the same time is bad on magnetic
disks because of the excessive disk seeks. So allow only 4 threads to
copy closures in parallel.
Aborted builds are now put back on the runnable queue and retried
after a certain time interval (currently 60 seconds for the first
retry, then tripled on each subsequent retry).
This removes the need for Nix's build-remote.pl.
Build logs are now written to $HYDRA_DATA/build-logs because
hydra-queue-runner doesn't have write permission to /nix/var/log.
