Add ConcurrentHashMap.KeySetView serializer

[bergander]

This PR adds support for serializing sets created by ConcurrentHashMap.newKeySet().

[theigl]

I just realized that this is not ideal. We are writing the keys twice. Once here as part of the map in the header and then again in the main write method of the CollectionSerializer. It would be enough to write the map because the keyset is only a view. Can you try to implement this without extending CollectionSerializer and implement Serializer directly? This should make the serialized payload significantly smaller.

[theigl]

writeHeader would simply become write and create would become read.

[bergander]

Thank you for pointing me in the right direction! PR is updated according to your comments.

[theigl]

That looks great now, thanks!