This patch prepares AIStateMachine for the use of AITimer together
with calls to set_state() from other threads. The extra problem
in this case is that the main-thread CAN start running the state
machine again (when the timer times out), while before it was
assumed to be idle until a thread called set_state.
This also takes into account that a thread might call set_state()
and then AGAIN call set_state() before the main thread gets the
chance to call idle() inbetween.