When a route's output is connected to multiple busses, re-establishing
port-connections on session-load calls Route::output_change_handler
multiple times.
Implicit solo counts are saved and restored by the route's SoloControl,
however the output_change_handler invalidated it on session load.
Since connections are restored iteratively, counting soloed downstream
routes will yield an incorrect result until all connections are
established.
This fixes the issue that implicit solo is lost on session-reload
in case a route has multiple connections with different up/downstream
solo states.
b90f057e6e