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.
This is somewhat bound to the "forcing the registration of a `CacheLoader` when having a `CacheWriter` registered" discussion, as are other bits of code, like the `putIfAbsent` code, but if we know there is a `CacheLoader` and we expect the `Cache` to remain consistent, this wouldn't be required... Other than maybe when we delegated to the `ResilienceStrategy.possiblyInconsistent`, in which case this probably should also delegate to it.