add extended wait interface

[BenjaminW3]

Add a variadic alpaka::wait::wait(waited1, ...) method to let the current thread wait for multiple objects at once.

It would also be useful to have a method alpaka::wait::wait(waiter, waited1, ...) to let a waiter wait for multiple objects at once.
This would require the two methods to have different names because then they can not be distinguished by the argument count anymore. What would be useful names?

Both methods could possibly be implemented by just sequentially calling the underlying standard wait methods because the events are one shot events only and do not trigger waits in the executing threads and therefore no deadlocks.