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.
These issues will be fixed when the `CacheLoader` is wired with #75 in though, right?
Now I think this are the cases where we said that a `CacheWriter` without a `CacheLoader` make no sense. It probably is worthwhile to figure out what we want to do about this, when #75 is done.