Avatar Performance Enhancements

Fixes issue #124 by reducing I/O using locale caches and Java 8 features wherever possible.

• Performance: Do not overwrite exactly equal bytes on disk

• Performance: userHashes locally persisted

  Uses local persistent store to speed up initialization of userHashes, hence subsequent Avatar retrieval needs less time.

• Performance: O(1) instead of O(n) for DirectoryCache::isEmpty()

• Java 7 -> Java 8