Auto merge of #2197 - RalfJung:round-robin, r=RalfJung

Auto merge of #2197 - RalfJung:round-robin, r=RalfJung

make Miri's scheduler proper round-robin

When thread N blocks or yields, we activate thread N+1 next, rather than always activating thread 0. This should guarantee that as long as all threads regularly yield, each thread eventually takes a step again.

Fixes the "multiple loops that yield playing ping-pong" part of https://github.com/rust-lang/miri/issues/1388.
`@cbeuw` I hope this doesn't screw up the scheduler-dependent tests you are adding in your PR.