Provide a way to iterate through a cache

[whitfin]

Although I wouldn't recommend this as a heavily-used operation, it does make sense to have a way to iterate the cache (similar to Redis' SCAN command family).

It'd be neat if there were a function of Cachex.stream(cache, options) which can be used to iterate a cache in such fashion:

:my_cache |> Cachex.stream! |> Enum.to_list

This would allow various filtering and whatnot, in order to allow people to filter out cache values and operate on them appropriately. For example if you wanted to delete all keys beginning with a:

list_of_a =
  :my_cache
  |> Cachex.stream!
  |> Enum.filter(fn{ key, value } ->
        String.starts_with(key, "a")
      end)
  |> Enum.to_list

Cachex.execute(:my_cache, fn(worker) ->
  Enum.each(list_of_a, &(Cachex.del(worker, &1))
end)