CopyOnWrite open addressing linear probing map/set

[franz1981]

Due to netty/netty#10530
I have used a copy on write approach with the existing IdentityHashMap: I believe a smarter approach would use the rehashing/compacting phases of a such maps on put/remove to improve the performance during modifications while saving tons of wasted memory too.
Hope to have some time to implement these...if not, please feel free to pick this one :)