Added a timeout parameter to semaphore::wait(), which defaults to no
timeout.

semaphore:wait() is now a boolean, just like trywait(), and likewise can
return false when the semaphore has not actually been decremented but
rather we had a timeout.

Because we need the mutex again after the wait, I replaced the "with_lock"
mechanism by the better-looking lock_guard and mutex parameter to
wait_until.