model: split THREAD_* processing into process_thread_action()

model: split THREAD_* processing into process_thread_action()

The THREAD_* specific functionality can easily be in its own function.

Additionally, this patch:

* performs the trivial rearrangement of moving this code inside the
  work_queue loop, for consistency

* returns a "synchronized" status, so that THREAD_FINISH/THREAD_JOIN can be
  hooked into the work_queue update loop later