increase the sleep semaphore threshold

Also remove the negation of the condition (!> equals <=).

We currently use the semaphore of our sleep strategy very greedy.
This is due to skipping the semaphore's V() operation if we are sure
that it does not kill our progress guaranty.

When scheduling new work from within the runtime we skip the wakeup if
we observe nobody sleeping. This is fine in terms of progress and
limits the amount of atomic operations on global state to a minimum.

Using a threshold of 0 (observe nobody sleeping) however introduces a
race between inserting new work and going to sleep which harm the
latency when the worker goes to sleep and is not notified about the
new work.

This race is common and can be observed in the pulse micro benchmark.
A emper with a threshold of 0 shows high latency compared to using
an io-based sleep strategy or when increasing the threshold.

$ build-release/eval/pulse | python -c "<calc mean>"
Starting pulse evaluation with pulse=1, iterations=30 and utilization=80
mean: 1721970116.425

$ build-increased-sem-threshold/eval/pulse | python -c "<calc mean"
Starting pulse evaluation with pulse=1, iterations=30 and utilization=80
mean: 1000023942.15

$ build-pipe-release/eval/pulse | python -c "<calc mean>
Starting pulse evaluation with pulse=1, iterations=30 and utilization=80
mean: 1000030557.0861111

$ build-pipe-no-completer/eval/pulse | python -c "<calc mean>"
Starting pulse evaluation with pulse=1, iterations=30 and utilization=80
mean: 1000021514.1805556

I could not measure any significant overhead due to the more atomics
on my 16 core machine using the fs-eval on a SSD.

$ ./eval.py -r 50 -i emper-vanilla emper-inc-sem-threshold emper-pipe emper-pipe-no-completer
...
$ ./summarize.py results/1599f44-dirty-pasture/<date>/ -f '{target}-{median} (+- {std})'
duration_time:u:
emper-vanilla-0.202106189 (+- 0.016075981812486713)
emper-inc-sem-threshold-0.2049344115 (+- 0.015348506939891596)
emper-pipe-0.21689131 (+- 0.015198438371285145)
emper-pipe-no-completer-0.1372724185 (+- 0.005865720218833998)