Use a more contention-friendly intern table

[toddjonker]

The current intern tables are generally WeakHashMaps with extremely coarse-grained locks. This should instead use a ConcurrentHashMap with weak-pointer support.

(Longer-term, we should investigate using our own persistent HAMT. I bet there's ways to make that more performant than Java's built-in hashes.)

[toddjonker]

My work in progress is at https://github.com/toddjonker/fusion-java/tree/intern-map-new

I would like to have benchmark data before merging the change, so this is blocked by #158