xua_lm: Add S_INACTIVE state and improve state transitions
The S_INACTIVE state will be needed once we support administrative state block in role ASP/(active)IPSP.
It is also useful now since sometimes we just want to leave an asp in that state without any timer running (eg. asps in override which are not active until the active one goes inactive).
Adding the S_INACTIVE state also allows transitioning states in role SG, since we now have somewhere to go instead of leaving it alwas as S_IDLE.
The S_INACTIVE state also allows staying in that state until the ASP ACT ACK is received, moment at which the ASP is considered ACTIVE according to specs.
