Reset subscription client on metadata update

[acogoluegnes]

A subscription is always cancelled on a metadata update (stream deleted or Erlang process of the stream member restarted), so there is no guarantee the consumer will keep the same client instance (= connection). We're better off setting it to null, which set the SAC active flag to false as well. This avoids also an unnecessary warning when the consumer is re-activated (it thinks it's already active).