Revisit implementation of memory session

The addition of a new exception kind in the liveness check (WrongThreadException) has introduced some asymmetry in the code, as we try not to allocate exceptions in the fast path, but we currently only do that for IllegalStateException.

Furthermore, with the addition of non closeable views, the implementation of memory sessions has not aged well - some mechanism has been added (Scoped::sessionImpl) to obtain the "true" session hidden behind a view (as we need this session for the liveness check in ScopedMemoryAccess). A better implementation is possible, by factoring stateful parts into a common State class that can be used by all sessions (including views).