like a normal semaphore, but with awaitable acquire actions. this is
primarily intended as an intermediate concurrency limiting device in
the Worker code, but it may find other uses over time. we do not use
std::counting_semaphore as a base because the counter of that is not
inspectable as will be needed for Worker. we also do not need atomic
operations for cross-thread consistency since we don't have multiple
threads (thanks to kj event loops being confined to a single thread)
Change-Id: Ie2bcb107f3a2c0185138330f7cbba4cec6cbdd95
