* add event_scheduler_impl.h and properly override its base class
* add urandom_entropy_src.cpp, move there logic from header
* add urandom_entropy_src.cpp to CMakeLists.txt
* add unique_ptr per impl: one for both EventSchedulerImpl and
EventAdderImpl each
* wrap a call in reinterpret_cast and reformat code
* add missing includes (best effort)
