need to keep state variables on both levels,
since the session manager (lifecycle) "opens" the session
for external access by starting the dispatcher; it may well happen
thus that the session starts up, while the *session subsystem*
is not(yet) started
"command dispatching" == the public session interface
so we'll better implement this important causal link directly,
instead of some obscure trickery with lifecycle events.
lib::Depend<TY> works as drop-in replacement for lib::Singleton<TY>
This changeset removes the convoluted special cases like
SingletonSub and MockInjector.
