[Runtime] use a POSIX semaphore to suspend/wakeup workers

To prevent deadlocks where all workers are going to sleep and new work
arrives from without emper we always increment the wakeup semaphore
unless we observe its value as > worker count.
If the semaphore value is bigger than worker count it is guarantied that
at least on worker will not block and will iterate a second time
in its dispatchLoop observing the new work.