This class was initially in Host because its implementation used to be
very OS-specific. However, with C++11, it has become a very simple
std::condition_variable wrapper, with no host-specific code.
It is also a general purpose utility class, so it makes sense for it to
live in a place where it can be used by everyone.
This has no effect on the layering right now, but it enables me to later
move the Listener+Broadcaster+Event combo to a lower layer, which is
important, as these are used in a lot of places (notably for launching a
process in Host code).