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.
Well yes and no. If the SoR is only ever accessed through the `Cache`, then yes it'd remain atomic (though, yes, would also mean 2 ops on the SoR) as the `Cache` would provide the locking & exclusion... If it isn't only accessed through the `Cache`, then not only doesn't it matter I think, but the whole approach becomes sketchy as it gets...