Two fixes for source sets

* If a process is in the predecessor set, prefer it over other processes when we
  want to reach a later instruction of it in a swapping.  Can be improved with
  'delaying-the-choice' (having *all* selections as possible and later fixing
  them, using additional required interleavings or finally chance).

* Use max of clock vectors of B & C instead of intermediate (B) clock vector
  when swapping B & C is not allowed due to sleep sets but swapping B & C
  together before A might later be possible.

Commited with a test that has no sleep set blocked interleavings.