Change read/command to command/2/3

`read` is deprecated in favor of `command/2` and `command/3` that now always returns a
return value per actor. In this way, the old `read/2` and `command/3` can be
implemented on top of the new command implementation.

Note: the error `actor_not_active` is no longer possible to get, instead
you will only get an empty list back from command/2/3 if there are no
actors.