* [REF-3184] Raise exception when encountering nested `async with self` blocks
Avoid deadlock when the background task already holds the mutation lock for a
given state.
* [REF-3339] get_state from background task links to StateProxy
When calling `get_state` from a background task, the resulting state instance
is wrapped in a StateProxy that is bound to the original StateProxy and shares
the same async context, lock, and mutability flag.
* If StateProxy has a _self_parent_state_proxy, retrieve the correct substate
* test_state fixup
0845d2ee76