Connectors may not transfer data after a system swap

Description

Consider a simple system of the kind:

head
├── a
└── b

with a connector a.y --> b.x.

We now swap sub-system b for c, with the same API.

During the next execution of head, if inputs have not changed, the new connector a.y --> c.x will fail to transfer data. The root cause is that system a, regarded as clean, will not be recomputed (as expected), but more importantly, output port a.y will be tagged as clean as well.

Here, we want to force data transfer since the connector is new and has never transmitted data, regardless of the status of port a.y.