Race condition in ipc.rs when RunOneOutcome::ThreadWaitNotification is generated

[tomaka]

There are two races there:

• New notifications can be added to the queue between when we try to resume the thread and when we add it to the queue.
• Once a thread is in the queue, it must by definition be unlocked. However, the thread object stays alive for a small amount of time after being inserted, and it is possible that the queue gets accessed in between.