Pass ContinuationOptions to channels and synchronization primitives

[timcassell]

By default, channels and async synchronization primitives continue on the captured context. We can add overloads for users to configure that continuation behavior after the new options are merged in with #479.

[timcassell]

I realized that the returned Promise could be completed before it is awaited, possibly making the continuation execute on a different context than the options. To fix this, we should instead pass a bool continueOnCapturedContext flag, and if users need it, they can use ConfigureAwait.