Waitset member allocation use rmw types (#190)
Waitset member allocation use rmw types (#190)

* allocate memory for rmw types and not rcl types in waitset RMW storage

* zero initialize memory on resize and clear

* add missing void * cast to make windows happy

* set mem after checking successful alloc and use right type for rmw storage

* another instance of cast to (rmw_ ## Type ## _t *) instead of (void *)

ros2/rcl,ros2/rcl