Optimization Idea: Skipping synchronization if thread not changed.

[Dandielo]

Sometimes we might have a situation where the Awaitable responsible to move a task to a different execution thread is already in the "ready" state, skipping all the necessary work. It would be great to notify functions like wait_for that there never was a thread change and the synchronization can be minimalized. (For example, not invoking the futex call on Unix / Android systems)