Optional upper limits for RAM usage

[kpcyrd]

One of rebuilders regularly dies due to low memory pressure, even though it has 64gb ram and 128gb swap.

This server is also used by other people, so this is quite the inconvenience.

I'd rather have an upper limit on the nspawn conatiner and have the build fail if this limit is exceeded. I think the BAD status is well deserved for those packages.