SI-7775 Avoid iterating over System.getProperties 

I don't know why it didn't use my commit message... This is for visibility in case it's useful. It was fun just reading the code. Oh, and helpfully I held off on a test until someone said this was what they wanted.

    Just when you want to iterate over system properties in search
    of a setting, someone else wants to configure their module by
    setting a property.
    
    Avoid ConcurrentModificationExceptions by iterating over property
    names only, which are not backed by the underlying map.