m-default-nodes: wait properly for nodes and ports

Device nodes and their ports may appear some time after devices.
Deviceless nodes may also appear.

If m-default-nodes skipped re-evaluations because some node ports were
not ready, it must retry when ports appear, because everything might be
ready then.

Rescan also when potential sink/source candidate nodes are changed.

Fixes default nodes sometimes pointing to non-existent nodes.
7 jobs for default-nodes-fix in 2 minutes and 10 seconds (queued for 1 second)
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