VTOL attitude controller: fix pusher support in hover

[sfuhrer]

Pusher assist in position control is broken currently. Reason for it is that _pusher_throttle is only updated every time https://github.com/PX4/Firmware/blob/69475a172bf743fb4b735d5e5595d8a23f2dba66/src/modules/vtol_att_control/vtol_att_control_main.cpp#L420 is executed, and otherwise _pusher_throttle=0 is set. And update_mc_state is only updated when mc_virtual_attitude_sp is updated.

Also reproducable in SITL (unsteady actuator_controls(3), see below)

[sfuhrer]

Test flown with standard VTOL, works well.
https://review.px4.io/plot_app?log=39810ece-53c9-4137-9fb8-617a519781de

[RomanBapst]

@dagar @sfuhrer Me and Silvan agreed that this part needs to be re-implemented cleanly, e.g. update_vtol_state() should be a pure state machine and not involve setting any variables like the pusher throttle.
I will merge this as it's tested and the current state is broken.