ESTOP should clear any internal queue

After estop is triggered, the machine state is set to homing_state_ = GCodeMachineControl::HomingState::NEVER_HOMED;
and motors are shutdown. Segments could still be enqueued in the planner or in worse in the PRU. 

We should enforce a clear state after that or an estop clear could lead to machine executing an unfinished PRU segment (at some undefined speed).
