Add task pinning #6933

A pinned task is one that only executes on a specific scheduler. Pinned tasks contain a SchedHandle and when they are scheduled they are sent as a message to their home scheduler.

This is used for 1:1 scheduling and the platform thread.

Note that this has subtle interaction with I/O affinity, e.g. if a pinned task is doing I/O using a handle that was created on a different scheduler it may need to bounce over to the I/O scheduler temporarily.