Prittyfied and simplified the local store API.

These changes include:

1. Extracting the specific request types from the `LocalStoreRequest`
   into separate classes. This way an operation can be isolated and
   is easier to overview.

2. Hiding the specific request implementations within the package.
   These requests are only intended to be used by the `LocalStore`
   class, hence there is no apparant reason to expose them.

3. Simplyfieing the `Store` API by not returning any previous values
   when deleting or overwriting them (applies to the "remove" and
   "set" operations). These would be essential features of a REST
   API, but the local store API isn't ment to be REST-ful.