Resource manager cleanup (#4732)

Resource manager cleanup (#4732)

This PR performs some general cleanup in and around the ResourceManager class. In detail, it does

remove --hard-limit (we decided to always leave the solver in a usable state, i.e. always do a soft limit),
remove --cpu-time (we decided to always use wall-clock time for time limiting)
replace old gettimeofday-based Timer by new std::chrono-based WallClockTimer
clean up the logic around beginCall() and endCall()