send_deferred is incorrect for SingleThreaded tasks

[brson]

When send_deferred sees a task to wake up it calls enqueue_blocked_task to reschedule the work, but enqueue_blocked_task assumes that some other scheduler will come along to steal the work. For tasks spawned with SingleThreaded scheduling though, the scheduler doesn't participate in work stealing, so if the task then goes on to block, the other task will never wake up.

[alexcrichton]

This will be closed by #10965

[alexcrichton]

send_deferred is now (mostly) gone