score: Simplify _Thread_Change_priority()

The function to change a thread priority was too complex.  Simplify it
with a new scheduler operation.  This increases the average case
performance due to the simplified logic.  The interrupt disabled
critical section is a bit prolonged since now the extract, update and
enqueue steps are executed atomically.  This should however not impact
the worst-case interrupt latency since at least for the Deterministic
Priority Scheduler this sequence can be carried out with a wee bit of
instructions and no loops.

Add _Scheduler_Change_priority() to replace the sequence of
  - _Thread_Set_transient(),
  - _Scheduler_Extract(),
  - _Scheduler_Enqueue(), and
  - _Scheduler_Enqueue_first().

Delete STATES_TRANSIENT, _States_Is_transient() and
_Thread_Set_transient() since this state is now superfluous.

With this change it is possible to get rid of the
SCHEDULER_SMP_NODE_IN_THE_AIR state.  This considerably simplifies the
implementation of the new SMP locking protocols.
score: Add and use _Scheduler_default_Schedule()
score: Add and use _Scheduler_SMP_Start_idle()
score: Make _Thread_queue_Requeue() static

This function is only used by _Thread_Change_priority().  Make it static
to avoid the function call overhead in the performance critical function
_Thread_Change_priority().
score: Split SMP scheduler enqueue function

Extract code from _Scheduler_SMP_Enqueue_ordered() and move it to the
new function _Scheduler_SMP_Enqueue_scheduled_ordered() to avoid
untestable execution paths.

Add and use function _Scheduler_SMP_Unblock().