Make span duration rendering optional and turn it off by default

[oli-obk]

See https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Changing.20formatting.20of.20rustc.20logs for some rustc discussions. Basically we don't need them in rustc, like ever, as they just make our log files hard to diff, so now they are configurable