Connection manager enforces non-blocking disconnect() behavior, yet in
case CONN_MGR_IF_NO_AUTO_DOWN flag is not set, it'd put the interface
down right after, disrupting the disconnect process.
As putting the interface down can be handled in the corresponding event
handler as well, when the interface is actually disconnected, remove the
conn_mgr_conn_if_auto_admin_down() call from conn_mgr_if_disconnect().
To make this work with persistence flag, introduce a new internal flag
indicating that the interface is in active disconnect.
Finally, since it isn't really necessary that disconnect() API call is
non-blocking, remove that requirement.
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
011f2bc458