Issue 39

There may still be some rough edges here. In particular, it feels like `CacheWriter.write(K, V)` needs to be split into insert and update, and return a value. Also, an attempt was made to whip compute/write functions for bulk CRUD methods into shape, to blaze the trail for fixing #110... not super convinced they are correct now though.
Hmm, I suppose it could be combined with a `CacheLoader` call, but then it won't be an atomic operation wrt SOR, would it? Does it even matter?