[threading] Split unwind state into sync and async.

[threading] Split unwind state into sync and async.

Having a single unwind state means writes to it must be synchronized between sync and async suspend. We can't
have the state be broken as it could lead to bad unwinding or GC marking.

The original solution was to have an additional state in the self suspend path that signals it's writing to the
thread state and thus any async suspend must give up and let it finish. This ended up been overly complicated
as this requires two additional states instead.

By having a pair of unwind states both can write concurrently without the fear of clashing. This makes fetching the state
a little bit trickier but worth the trouble.

The simplified design has a much smaller state space, which is easier to reason about.