Automatically clean lintchecks shared target dir (will force clippy to recheck sources) if
the clippy binaries are older than the lintcheck logs.
Also update lintcheck logs
Use rayon to figure out the threadcount and half that for core count.
For each core, create a target dir that is used.
Otherwise, when running multiple clippys with the same target-dir, cargo would lock the dir and prevent parallelism.
This way we can run multiple clippys at the same time (on root crates) but we sacrifice cache-hits (when we already cargo-checked crate-deps).
