Async_kernel.ConditionSourceAsync's implementation of the standard notion of a "condition" variable.
This is analogous to OCaml's Condition module. The main guarantee that a condition variable provides is that a call to signal (or broadcast) after a call to wait will be seen by the waiter.
Unlike the use of condition variables in ordinary threaded programs, Async condition variables do not require a mutex, since Async programs are cooperatively threaded.