This is required to be compatible with cloud.google.com/go/storage that
depends on an incompatible version of protobuf

The 'clientKind' argument is 'actor', not the actual actor type.

This new approach makes it a lot easier to implement workers and consumers and _test_ them.
It is still fresh and will probably need some ironing.
Note that this first implementation does not allow detached processing in order
to simplify everything.

In case of error in Read() and Write(), the sftp connection was never released, blocking
everything.