Remove custom TUser implementation

- Since we don't really have a good way to persist extra attributes for a custom TUser class (yet), let's remove the TUser parameter from the store and instead just focus on supporting CassandraUser (which we also remove the TKey parameter from and just use a Guid by default for now)
Remove IQueryable support

- We'll *probably* never expose users via IQueryable since C* doesn't support ad-hoc querying, so just remove that interface/property