Add waitFor to djinni::Future

[jb-gcx]

My project needs the ability to wait on a future with a timeout, mostly to make sure unit tests don't hang forever if unexpected things happen. I thought this might be useful for others as well.

In waitImpl, I used the same scheme as std::condition_variable::wait_until for dealing with clocks. This way it should be easy to add djinni::Future::waitUntil if needed.