`mktempdir` should support `cleanup=true` by default

[ericphanson]

to match Julia base on 1.3+. This would be something like adding atexit(() -> rm(dir; recursive=true)) inside mktempdir().

[rofinn]

Hmm, I suppose we can provide that very basic fallback for now. I suspect it won't work properly for all filesystems. For example, the base code:

1. Maintains a proper registry of temp paths to cleanup
2. Handles locking and unlocking the registry for concurrent updates
3. Handles chmod permissions

I don't think this is applicable for S3Path, but it'll likely require overloading a custom temp_cleanup method.