Switching Ray development to C++14 or C++17

Dear all,

currently we are using C++11 for Ray development but we are planning to switch to C++14 or C++17 so we can take advantage of new features like std::optional, string_view, variant, move in lambda capture, make_unique, better inference for auto, or structure binding.

Since we are using manylinux2014 wheels, switching to C++14 or 17 will have no impact on users of the Ray binaries. For users who build Ray from source, switching to C++14 should be safe since other widely used OSS projects like TensorFlow, pytorch or LLVM have already made the switch and C++14 is widely supported now.

The question is if we can directly go to C++17 and for this we want to collect feedback from the community. Please let us know if you are using a custom compiled Ray on a platform that doesn’t support C++17!