fix sleep timeout in scheduler

In the case that the current time equals exactly the timeout of
a `sync/timeout`, the Racket process could get stuck (using no CPU)
instead of continuing as it should.

How did we not find this before? Why am I suddenly able to replicate
the problem (i.e., hitting exactly the target timeout in the secheduler
at the millisecond granularity)?

Merge to v6.1