Use ConcurrentHashMap.newKeySet() as newConcurrentHashSet

[turbanoff]

Method ConcurrentHashMap.newKeySet() was introduced in java 8.
It can be used to create concurrent hash set implementation. It can be faster that Collections.newSetFromMap as it removes one indirection level - https://stackoverflow.com/questions/32054517/concurrenthashmap-newkeyset-vs-collections-newsetfrommap

[cgdecker]

I think that the advantages of using newKeySet() are likely extremely minimal, and as such it's not worth introducing an additional diff between the JRE and Android flavors of Guava for this. That said, I could definitely see doing this at whatever point in the future we can use Java 8 APIs everywhere in Guava, so I'm leaving this open.