make credit waiting less brittle
We were relying on running out of credit for all slaves
*simultaneously*, which requires in-depth knowledge of the credit flow
logic and that no other credit-requiring messages are sent to a slave
prior to this.

Fortunately, since we are running in a fresh separate process we can
simply handle *any* credit bumping message and *any* DOWN message.

As a bonus we can revert to making the type of the bump msg opaque.