ObjectBox C/C++ 0.9.1

• C++ interface improvements:
  • "Box::getOptional()" overloads returning std::optional
  • "Box::put()" overloads taking vectors of std::unique_ptr and std::optional
  • Query methods: find(), findIds(), count(), remove(), ...
  • Ensure double-free can't happen (added explicit copy & move constructors)
• Fixed Windows exported symbols - recently added APIs were missing
• New "obx_cursor_put_object4()" overload taking PutMode as an argument
• Make *_close() functions consistently accept nullptr

See also the Generator companion project.
(https://objectbox.io/introducing-objectbox-generator-plus-c-api/)