Clean up constants that are multiply defined in various places.

[staffantj]

This change is fine for now, but it'd be a good idea to make another pass through the code (in a separate changelist/PR) to get rid of these constants sprinkled around in the code and use the functions in missing_values.h instead.

Originally posted by @jack-pappas in https://github.com/rtosholdings/riptide_cpp/pull/66#r753224227