* Update initCancellability documentation and implementation to be aligned with current invariants
    * Make parentHandle non-volatile and ensure there are no races around it
    * Establish new reusability invariants
      - Reusable continuation can be used _only_ if it's state is not REUSABLE_CLAIMED
      - If it is, spin-loop and wait for release
      - Now the parent is attached to reusable continuation only if it was suspended at least once. Otherwise, the state machine can return via fast-path and noone will be able to release intercepted continuation (-> detach from parent)
      - It implies that the parent is attached after trySuspend call and can be concurrently reused, this is where new invariant comes into play

Fixes #2564

    * Leverage the fact that it's non-atomic and do not check it for cancellation prematurely. It increases the performance of fast-path, but potentially affects rare cancellation cases
    * Fix AwaitContinuation that has parentHandle == null but is not reusable

…belong to the current instance